:root {
  --bg: #0f1412;
  --panel: #18211d;
  --panel-border: #2a3a33;
  --key: #24302b;
  --key-hover: #2e3d36;
  --fn: #31443b;
  --op: #1f6b4a;
  --op-hover: #26845b;
  --equals: #2f9e6a;
  --text: #eef6f1;
  --muted: #93a89d;
  --accent: #7ee0ad;
}

* {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  min-height: 100vh;
  display: grid;
  place-items: center;
  font-family: Inter, ui-sans-serif, system-ui, -apple-system, Segoe UI, sans-serif;
  background:
    radial-gradient(circle at top, #1c3a2c 0%, transparent 42%),
    var(--bg);
  color: var(--text);
}

.app {
  width: min(420px, calc(100vw - 32px));
  padding: 28px 0;
}

.brand {
  margin-bottom: 18px;
}

.eyebrow {
  color: var(--accent);
  letter-spacing: 0.14em;
  text-transform: uppercase;
  font-size: 12px;
  margin-bottom: 6px;
}

h1 {
  font-size: 28px;
  font-weight: 650;
}

.calculator {
  background: linear-gradient(180deg, #1c2621, var(--panel));
  border: 1px solid var(--panel-border);
  border-radius: 28px;
  padding: 20px;
  box-shadow: 0 24px 50px rgba(0, 0, 0, 0.35);
}

.display {
  min-height: 108px;
  margin-bottom: 18px;
  padding: 16px 14px;
  border-radius: 18px;
  background: #101714;
  text-align: right;
  overflow: hidden;
}

.expression {
  min-height: 22px;
  color: var(--muted);
  font-size: 15px;
  word-break: break-all;
}

.result {
  margin-top: 8px;
  font-size: 42px;
  line-height: 1;
  font-weight: 650;
  letter-spacing: -0.03em;
}

.keys {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 10px;
}

.key {
  height: 64px;
  border: 0;
  border-radius: 16px;
  background: var(--key);
  color: var(--text);
  font-size: 22px;
  font-weight: 600;
  cursor: pointer;
  transition: transform 0.08s ease, background 0.15s ease;
}

.key:hover {
  background: var(--key-hover);
}

.key:active {
  transform: scale(0.96);
}

.key.fn {
  background: var(--fn);
  color: #d7efe3;
}

.key.op {
  background: var(--op);
}

.key.op:hover,
.key.equals:hover {
  background: var(--op-hover);
}

.key.equals {
  background: var(--equals);
}

.key.wide {
  grid-column: span 2;
}

@media (max-width: 420px) {
  .result {
    font-size: 36px;
  }

  .key {
    height: 58px;
    font-size: 20px;
  }
}