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

:root {
  --bg: #1c1c1e;
  --surface: #2c2c2e;
  --surface-2: #3a3a3c;
  --btn-num: #2c2c2e;
  --btn-op: #3a3a3c;
  --btn-fn: #505052;
  --btn-eq: #ff9f0a;
  --text: #f5f5f7;
  --text-soft: #8e8e93;
  --orange: #ff9f0a;
  --radius: 50%;
}

body {
  min-height: 100vh;
  display: flex;
  align-items: center;
  justify-content: center;
  background: #111113;
  font-family: "Outfit", sans-serif;
}

.calc-wrap {
  width: 360px;
  overflow: hidden;
  border-radius: 36px;
  background: var(--bg);
  box-shadow: 0 40px 80px rgba(0, 0, 0, 0.7), 0 0 0 1px rgba(255, 255, 255, 0.05), inset 0 1px 0 rgba(255, 255, 255, 0.07);
}

.display-area {
  min-height: 170px;
  display: flex;
  flex-direction: column;
  align-items: flex-end;
  justify-content: flex-end;
  gap: 6px;
  padding: 32px 28px 20px;
}

#expression {
  min-height: 22px;
  color: var(--text-soft);
  font-size: 16px;
  font-weight: 300;
  letter-spacing: 0.02em;
  text-align: right;
  word-break: break-all;
  transition: opacity 0.15s;
}

#display {
  color: var(--text);
  font-size: 64px;
  font-weight: 300;
  letter-spacing: -2px;
  line-height: 1;
  text-align: right;
  word-break: break-all;
  transition: font-size 0.15s;
}

#display.shrink {
  font-size: 42px;
}

#display.shrink-2 {
  font-size: 30px;
}

.buttons {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 12px;
  padding: 16px 18px 28px;
}

.btn {
  position: relative;
  aspect-ratio: 1;
  display: flex;
  align-items: center;
  justify-content: center;
  overflow: hidden;
  border: none;
  border-radius: var(--radius);
  cursor: pointer;
  font-family: "Outfit", sans-serif;
  font-size: 22px;
  font-weight: 400;
  user-select: none;
  transition: filter 0.08s, transform 0.08s;
  -webkit-tap-highlight-color: transparent;
}

.btn::after {
  content: "";
  position: absolute;
  inset: 0;
  border-radius: inherit;
  background: rgba(255, 255, 255, 0);
  transition: background 0.12s;
}

.btn:active::after {
  background: rgba(255, 255, 255, 0.12);
}

.btn:active {
  transform: scale(0.93);
}

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

.btn-op {
  background: var(--btn-op);
  color: var(--orange);
  font-size: 26px;
}

.btn-fn {
  background: var(--btn-fn);
  color: var(--text);
  font-size: 18px;
}

.btn-eq {
  background: var(--btn-eq);
  color: #fff;
  font-size: 28px;
  box-shadow: 0 4px 20px rgba(255, 159, 10, 0.35);
}

.btn-eq:active {
  filter: brightness(0.85);
}

.btn-zero {
  grid-column: span 2;
  aspect-ratio: unset;
  justify-content: flex-start;
  padding-left: 30px;
  border-radius: 40px;
}

@media (max-width: 420px) {
  body {
    align-items: stretch;
    background: var(--bg);
  }

  .calc-wrap {
    width: 100%;
    min-height: 100vh;
    display: flex;
    flex-direction: column;
    justify-content: flex-end;
    border-radius: 0;
  }
}