:root {
  --bg: #ffffff;
  --muted: #616161;
  --card: #f4f6f8;
  --accent: #1976d2;
  --accent-dark: #0d47a1;
  --success: #e3f2fd;
  --radius: 10px;
  --pad: 20px;
  font-family: 'Inter', sans-serif;
}

body {
  margin: 0;
  background: var(--bg);
  color: #222;
  line-height: 1.45;
  padding: 36px;
}

header {
  display: flex;
  align-items: center;
  gap: 16px;
  margin-bottom: 18px;
}

.logo {
  width: 56px;
  height: 56px;
  border-radius: 6px;
  background: #000;
  color: #29b6f6;
  display: grid;
  place-items: center;
  font-weight: 700;
  font-size: 26px;
}

h1 { margin: 0; font-size: 32px; }
p.lead { margin: 8px 0 28px; color: var(--muted); font-size: 16px; }

.container { max-width: 980px; margin: 0 auto; }
.card {
  background: var(--card);
  border-radius: var(--radius);
  padding: 18px;
  margin: 18px 0;
  display: flex;
  align-items: center;
  gap: 18px;
  box-shadow: 0 1px 0 rgba(16, 24, 40, 0.03);
}

.card .left {
  flex: 1;
  display: flex;
  align-items: center;
  gap: 12px;
  flex-wrap: wrap;
}

label { color: #333; }
input[type="number"] {
  width: 120px;
  padding: 10px;
  border-radius: 6px;
  border: 1px solid #d7d9db;
  background: #fff;
}

.btns { display: flex; flex-direction: column; gap: 8px; align-items: flex-end; }
.btn {
  background: var(--accent);
  color: #fff;
  padding: 12px 20px;
  border-radius: 8px;
  border: 0;
  font-weight: 700;
  cursor: pointer;
}
.btn:hover { background: var(--accent-dark); }
.clear {
  background: #bdbdbd;
  color: #fff;
  padding: 10px 14px;
  border-radius: 8px;
  border: 0;
  cursor: pointer;
}
.result {
  width: 130px;
  padding: 10px;
  border-radius: 6px;
  border: 1px solid #d7d9db;
  background: var(--success);
  font-weight: 600;
}

.tips {
  margin: 20px 0;
  padding: 12px;
  border: 1px solid #eee;
  border-radius: 6px;
  color: var(--muted);
  font-style: italic;
  background: #fff;
}

h2 { margin: 0 0 12px; font-size: 20px; color: #0b3a63; }
footer { margin-top: 28px; text-align: center; color: var(--muted); font-size: 14px; }

@media (max-width: 720px) {
  .card { flex-direction: column; align-items: stretch; }
  .btns { flex-direction: row; justify-content: space-between; }
  input[type="number"], .result { width: 100%; }
}