:root{
  --bg: #f0f4f9;
  --panel: #ffffff;
  --accent: #2563eb;
  --accent-light: #3b82f6;
  --muted: #64748b;
  --text: #0f172a;
  --glass: rgba(15,23,42,0.04);
  --shadow-sm: 0 2px 8px rgba(0,0,0,0.04);
  --shadow-md: 0 4px 16px rgba(0,0,0,0.06);
  --shadow-lg: 0 8px 24px rgba(0,0,0,0.08);
}
*{box-sizing:border-box}
html,body{height:100%;margin:0;font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, 'Hiragino Kaku Gothic ProN', 'Noto Sans JP', 'Yu Gothic', system-ui, sans-serif;background:var(--bg);color:var(--text);-webkit-font-smoothing:antialiased;-moz-osx-font-smoothing:grayscale}
.container{max-width:900px;margin:0 auto;padding:20px}
.header-content{text-align:center;width:100%}
h1{margin:0 0 8px;font-weight:700;font-size:2rem;font-family:'Noto Sans JP', 'Inter', system-ui, sans-serif;background:linear-gradient(135deg,#2563eb,#7c3aed);-webkit-background-clip:text;-webkit-text-fill-color:transparent;background-clip:text}
.subtitle{color:var(--muted);margin:0;font-size:15px;font-weight:500}
.top{display:flex;align-items:center;justify-content:center;margin-bottom:24px;padding:20px 0}
.main-panel{display:grid;grid-template-columns:260px 1fr;gap:24px;align-items:start}
.card{width:100%;aspect-ratio:1;max-width:180px;margin:0 auto;border-radius:20px;background:linear-gradient(135deg,#ffffff 0%,#f0f9ff 100%);display:flex;align-items:center;justify-content:center;font-size:64px;margin:12px auto 16px;box-shadow:var(--shadow-lg),0 0 0 1px rgba(59,130,246,0.1);border:2px solid rgba(59,130,246,0.1);color:var(--accent);transition:all .3s cubic-bezier(.2,.9,.2,1);cursor:pointer}
.card:hover{transform:translateY(-4px) scale(1.03);box-shadow:0 16px 40px rgba(37,99,235,0.15),0 0 0 2px rgba(59,130,246,0.2)}
.card-icon{display:inline-block;transition:transform .3s ease}
.card:hover .card-icon{transform:rotate(10deg) scale(1.1)}
.primary{width:100%;display:inline-flex;align-items:center;justify-content:center;gap:10px;padding:14px 24px;border:0;border-radius:12px;background:linear-gradient(135deg,#2563eb 0%,#1e40af 100%);color:white;font-weight:700;font-size:15px;cursor:pointer;box-shadow:0 8px 24px rgba(37,99,235,0.25);transition:all .2s ease;position:relative;overflow:hidden}
.primary::before{content:'';position:absolute;top:0;left:-100%;width:100%;height:100%;background:linear-gradient(90deg,transparent,rgba(255,255,255,0.2),transparent);transition:left .5s}
.primary:hover{transform:translateY(-2px);box-shadow:0 12px 32px rgba(37,99,235,0.35)}
.primary:hover::before{left:100%}
.primary:active{transform:translateY(0)}
.controls{display:flex;flex-direction:column;gap:8px;margin-top:16px}
.control-btn{width:100%;padding:10px 14px;border-radius:10px;border:1px solid rgba(15,23,42,0.08);background:white;color:var(--text);cursor:pointer;transition:all .2s ease;display:flex;align-items:center;gap:10px;font-size:14px;font-weight:500;box-shadow:var(--shadow-sm)}
.control-btn svg{flex-shrink:0;transition:transform .2s}
.control-btn:hover{background:linear-gradient(135deg,#f8fafc,#f1f5f9);border-color:rgba(59,130,246,0.3);color:var(--accent);transform:translateX(2px);box-shadow:var(--shadow-md)}
.control-btn:hover svg{transform:scale(1.1)}
.control-btn:active{transform:translateX(0) scale(0.98)}
.panel{background:white;border-radius:16px;padding:20px;margin-bottom:20px;box-shadow:var(--shadow-md);border:1px solid rgba(15,23,42,0.06);transition:all .3s ease}
.panel:hover{box-shadow:var(--shadow-lg);border-color:rgba(59,130,246,0.1)}
.panel h3{margin:0 0 16px;font-size:16px;font-weight:700;color:var(--text);display:flex;align-items:center;gap:8px}
.mode-panel{background:white;border-radius:16px;padding:18px;box-shadow:var(--shadow-md);margin-bottom:16px;border:1px solid rgba(15,23,42,0.06)}
.mode-label{display:block;font-size:14px;font-weight:700;color:#1e293b;margin-bottom:12px}
.mode-options{display:flex;gap:8px}
.mode-option{flex:1;display:flex;align-items:center;justify-content:center;padding:12px 8px;border-radius:10px;cursor:pointer;transition:all .25s cubic-bezier(.4,0,.2,1);border:2px solid #e2e8f0;background:white;position:relative;overflow:hidden}
.mode-option::before{content:'';position:absolute;inset:0;background:linear-gradient(135deg,#eff6ff,#dbeafe);opacity:0;transition:opacity .25s}
.mode-option:hover{border-color:#cbd5e1;transform:translateY(-2px);box-shadow:var(--shadow-sm)}
.mode-option input[type="radio"]{display:none}
.mode-option span{font-size:14px;color:#64748b;user-select:none;font-weight:600;position:relative;z-index:1;transition:color .25s}
.mode-option input[type="radio"]:checked + span{color:#2563eb;font-weight:700}
.mode-option:has(input[type="radio"]:checked){border-color:#2563eb;transform:translateY(-2px);box-shadow:0 4px 12px rgba(37,99,235,0.2)}
.mode-option:has(input[type="radio"]:checked)::before{opacity:1}
.difficulty-panel{background:white;border-radius:16px;padding:18px;box-shadow:var(--shadow-md);margin-top:16px;border:1px solid rgba(15,23,42,0.06)}
.difficulty-label{display:block;font-size:14px;font-weight:700;color:#1e293b;margin-bottom:12px}
.difficulty-options{display:flex;flex-direction:column;gap:6px}
.difficulty-option{display:flex;align-items:center;padding:10px 12px;border-radius:8px;cursor:pointer;transition:all .2s;border:1px solid transparent}
.difficulty-option:hover{background:#f8fafc;border-color:#e2e8f0}
.difficulty-option input[type="radio"]{margin:0;margin-right:10px;cursor:pointer;accent-color:#2563eb;width:18px;height:18px}
.difficulty-option span{font-size:14px;color:#475569;user-select:none;font-weight:500}
.difficulty-option input[type="radio"]:checked + span{color:#2563eb;font-weight:700}
.difficulty-option:has(input[type="radio"]:checked){background:linear-gradient(135deg,#eff6ff,#dbeafe);border-color:#2563eb}
.math{background:var(--panel);padding:14px;border-radius:10px;margin:8px 0;min-height:48px;border:1px solid rgba(15,23,42,0.04);box-shadow:0 6px 18px rgba(16,24,40,0.04)}
.hidden{display:none}
.history-panel{padding-top:0}
.history-header-wrapper{display:flex;align-items:center;justify-content:space-between;gap:12px;margin-bottom:16px}
.history-header-wrapper h3{margin:0}
.clear-history-btn{padding:8px 14px;border-radius:8px;border:1px solid rgba(15,23,42,0.08);background:white;cursor:pointer;transition:all .2s;display:flex;align-items:center;gap:6px;font-size:13px;font-weight:600;color:#64748b}
.clear-history-btn svg{flex-shrink:0}
.clear-history-btn:hover{background:#fee2e2;border-color:rgba(239,68,68,0.3);color:#ef4444;transform:translateY(-1px);box-shadow:var(--shadow-sm)}
#history{list-style:none;padding:0;margin:8px 0;display:flex;flex-direction:column;gap:12px}
.history-item{border-radius:12px;background:linear-gradient(135deg,#f8fafc 0%,#f1f5f9 100%);border:1px solid rgba(15,23,42,0.06);transition:all .25s cubic-bezier(.4,0,.2,1);overflow:hidden}
.history-item:hover{border-color:rgba(59,130,246,0.2);box-shadow:0 2px 8px rgba(59,130,246,0.08)}
.history-header{display:flex;justify-content:space-between;align-items:center;gap:12px;padding:12px 16px}
.history-problem{flex:1;cursor:pointer;transition:color .2s}
.history-problem:hover{color:#3b82f6}
.history-controls{display:flex;gap:4px;flex-shrink:0}
.history-toggle-btn,.history-delete-btn{width:28px;height:28px;border-radius:6px;border:1px solid rgba(15,23,42,0.1);background:#fff;cursor:pointer;display:flex;align-items:center;justify-content:center;font-size:14px;transition:all .2s;padding:0}
.history-toggle-btn:hover{background:#f1f5f9;border-color:rgba(59,130,246,0.3);color:#3b82f6}
.history-delete-btn{font-size:18px;line-height:1;color:#64748b}
.history-delete-btn:hover{background:#fee2e2;border-color:rgba(239,68,68,0.3);color:#ef4444}
.history-item .meta{font-size:11px;color:#94a3b8;padding:0 16px 8px 16px}
.history-content{max-height:0;overflow:hidden;transition:max-height .3s cubic-bezier(.4,0,.2,1);padding:0 16px}
.history-content.expanded{max-height:2000px;padding:0 16px 12px 16px}
.history-label{font-size:12px;font-weight:600;color:#475569;margin-bottom:6px}
.history-solution{padding:10px;background:#fff;border-radius:8px;border:1px solid rgba(15,23,42,0.06);font-size:14px}
.history-explanation{padding:10px;background:#fff;border-radius:8px;border:1px solid rgba(15,23,42,0.06);font-size:13px;line-height:1.6}
.spin{animation:spin 700ms cubic-bezier(.22,.9,.2,1)}
@keyframes spin{0%{transform:scale(1)}50%{transform:scale(1.06)}100%{transform:scale(1)}}

/* collapsible solution */
.solution.collapsible{max-height:0;min-height:0;overflow:hidden;transition:max-height .32s ease;padding:0;border:none;box-shadow:none}
.solution.expanded{max-height:420px;min-height:48px;padding:14px;border:1px solid rgba(15,23,42,0.04);box-shadow:0 6px 18px rgba(16,24,40,0.04)}

@media(max-width:768px){
  .container{padding:16px}
  .main-panel{grid-template-columns:1fr;gap:20px}
  .card{max-width:140px;font-size:56px}
  h1{font-size:1.75rem}
  .controls{flex-direction:column}
  .control-btn{width:100%}
  .mode-options{flex-direction:column}
  .mode-option{padding:12px}
}
@media(max-width:480px){
  .container{padding:12px}
  .card{max-width:120px;font-size:48px;margin:8px auto 12px}
  h1{font-size:1.5rem}
  .subtitle{font-size:13px}
  .primary{padding:12px 20px;font-size:14px}
  .panel{padding:16px;margin-bottom:16px}
  .panel h3{font-size:15px}
  .mode-panel,.difficulty-panel{padding:14px}
  .mode-label,.difficulty-label{font-size:13px}
  .mode-option span,.difficulty-option span{font-size:13px}
  .control-btn{padding:9px 12px;font-size:13px}
}


/* Explanation styling */
.explanation {
  max-height: 0;
  min-height: 0;
  overflow: hidden;
  transition: max-height 0.3s ease, padding 0.3s ease;
  padding: 0;
  background: transparent;
  border-radius: 8px;
  font-size: 0.95rem;
  line-height: 1.7;
  border: none;
  box-shadow: none;
}

.explanation.expanded {
  max-height: 2000px;
  min-height: 48px;
  padding: 20px;
  background: #f8fafc;
  border: 1px solid rgba(15,23,42,0.04);
  box-shadow: 0 6px 18px rgba(16,24,40,0.04);
}

.explanation p {
  margin: 0 0 12px 0;
}

/* Footer */
.footer{margin-top:48px;padding:32px 0;border-top:1px solid rgba(15,23,42,0.08);text-align:center}
.footer-content{max-width:600px;margin:0 auto}
.footer p{margin:0;color:#64748b;font-size:14px;line-height:1.6}
.footer-subtitle{font-size:13px;color:#94a3b8;margin-top:8px!important}

.explanation .step {
  margin: 16px 0;
  padding: 12px;
  background: white;
  border-left: 3px solid #3b82f6;
  border-radius: 4px;
}

.explanation .step-title {
  font-weight: 600;
  color: #1e40af;
  margin-bottom: 8px;
}