:root {
  --body-case: #2b2b2e;
  --body-case-edge: #3a3a3e;
  --screen-bg: #1a1206;
  --screen-glow: #ff9f1c;
  --screen-glow-dim: rgba(255, 159, 28, 0.35);
  --key-num: #e8e4dc;
  --key-num-text: #201f1d;
  --key-fn: #55545c;
  --key-fn-text: #f1efe9;
  --key-op: #3a3940;
  --key-op-text: #ff9f1c;
  --key-eq: #ff6b35;
  --key-eq-text: #1a1206;
  --shadow-deep: rgba(0, 0, 0, 0.55);
}

* {
  box-sizing: border-box;
}

body {
  margin: 0;
  min-height: 100vh;
  display: flex;
  flex-direction: column;
  align-items: center;
  justify-content: center;
  gap: 20px;
  background: radial-gradient(circle at 50% 20%, #3a3a40 0%, #1c1c1e 70%);
  font-family: 'Segoe UI', Roboto, -apple-system, sans-serif;
  padding: 24px;
}

.calc {
  width: 320px;
  background: linear-gradient(160deg, var(--body-case-edge), var(--body-case) 40%);
  border-radius: 22px;
  padding: 20px 18px 24px;
  box-shadow:
    0 25px 50px var(--shadow-deep),
    inset 0 1px 0 rgba(255, 255, 255, 0.06);
}

.calc__brand {
  display: flex;
  align-items: center;
  justify-content: space-between;
  padding: 0 4px 12px;
  color: #8a8a90;
  font-size: 11px;
  letter-spacing: 2px;
  font-weight: 600;
}

.calc__brand-dot {
  width: 6px;
  height: 6px;
  border-radius: 50%;
  background: var(--screen-glow);
  box-shadow: 0 0 6px 1px var(--screen-glow-dim);
}

.calc__screen {
  background: var(--screen-bg);
  border-radius: 10px;
  padding: 18px 16px 14px;
  margin-bottom: 18px;
  box-shadow:
    inset 0 2px 6px rgba(0, 0, 0, 0.6),
    inset 0 0 20px rgba(255, 159, 28, 0.04);
  min-height: 84px;
  display: flex;
  flex-direction: column;
  justify-content: flex-end;
  overflow: hidden;
}

.calc__history {
  font-family: 'Courier New', monospace;
  font-size: 13px;
  color: rgba(255, 159, 28, 0.45);
  min-height: 16px;
  text-align: right;
  white-space: nowrap;
  overflow: hidden;
  text-overflow: ellipsis;
}

.calc__display {
  font-family: 'Courier New', monospace;
  font-size: 40px;
  font-weight: 700;
  color: var(--screen-glow);
  text-align: right;
  text-shadow: 0 0 12px var(--screen-glow-dim);
  white-space: nowrap;
  overflow: hidden;
  text-overflow: clip;
  letter-spacing: 1px;
}

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

.key {
  border: none;
  border-radius: 12px;
  font-size: 18px;
  font-weight: 600;
  padding: 16px 0;
  cursor: pointer;
  transition: transform 0.06s ease, box-shadow 0.06s ease;
  box-shadow: 0 3px 0 rgba(0, 0, 0, 0.35);
}

.key:active,
.key.is-pressed {
  transform: translateY(3px);
  box-shadow: 0 0 0 rgba(0, 0, 0, 0.35);
}

.key:focus-visible {
  outline: 2px solid var(--screen-glow);
  outline-offset: 2px;
}

.key--num {
  background: var(--key-num);
  color: var(--key-num-text);
}

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

.key--op {
  background: var(--key-op);
  color: var(--key-op-text);
  font-size: 20px;
}

.key--eq {
  background: var(--key-eq);
  color: var(--key-eq-text);
  grid-column: span 2;
  font-size: 20px;
}

.key--zero {
  grid-column: span 1;
}

.howto {
  width: 320px;
  background: linear-gradient(160deg, var(--body-case-edge), var(--body-case) 40%);
  border-radius: 16px;
  padding: 18px 20px 20px;
  box-shadow:
    0 15px 30px var(--shadow-deep),
    inset 0 1px 0 rgba(255, 255, 255, 0.06);
  color: #cfced4;
}

.howto__title {
  margin: 0 0 10px;
  font-size: 13px;
  letter-spacing: 2px;
  text-transform: uppercase;
  color: var(--screen-glow);
  font-weight: 700;
}

.howto__list {
  margin: 0 0 14px;
  padding-left: 20px;
  font-size: 14px;
  line-height: 1.6;
}

.howto__list li + li {
  margin-top: 4px;
}

.howto__list strong {
  color: #f1efe9;
}

.howto__note {
  margin: 0;
  padding-top: 12px;
  border-top: 1px solid rgba(255, 255, 255, 0.08);
  font-size: 13px;
  line-height: 1.6;
  color: #a5a4ab;
}

.howto__note code,
.howto__note kbd {
  background: rgba(255, 159, 28, 0.12);
  color: var(--screen-glow);
  border-radius: 4px;
  padding: 1px 6px;
  font-family: 'Courier New', monospace;
  font-size: 12px;
}

@media (max-width: 360px) {
  .calc,
  .howto {
    width: 100%;
  }
  .calc__display {
    font-size: 32px;
  }
}