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

body {
  font-family: sans-serif;
  margin: 0;
  padding: 1rem;
  background: #f2f4f7;
  color: #1f2933;
}

main {
  max-width: 20rem;
  margin: 0 auto;
}

h1 {
  font-size: 1.25rem;
  text-align: center;
}

.rekenmachine {
  background: #fff;
  padding: 1rem;
  border-radius: 0.75rem;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.15);
}

.display {
  background: #1f2933;
  color: #fff;
  font-size: 2rem;
  text-align: right;
  padding: 0.75rem;
  border-radius: 0.5rem;
  margin-bottom: 1rem;
  overflow-x: auto;
  white-space: nowrap;
}

.knoppen {
  display: grid;
  grid-template-columns: repeat(4, 1fr);
  gap: 0.5rem;
}

.knoppen button {
  font-size: 1.25rem;
  min-width: 0;
  min-height: 3.5rem;
  padding: 1rem 0;
  touch-action: manipulation;
  border: 0;
  border-radius: 0.5rem;
  background: #e4e7eb;
  cursor: pointer;
}

.knoppen button:active {
  filter: brightness(0.9);
}

.knoppen .bewerking {
  background: #cbd2d9;
}

.knoppen .wissen {
  background: #f5b5b5;
}

.knoppen .gelijk {
  grid-row: span 3;
  background: #3e7bfa;
  color: #fff;
}

.knoppen .nul {
  grid-column: span 3;
}

@media (max-width: 400px) {
  body {
    padding: 0.5rem;
  }

  .rekenmachine {
    padding: 0.75rem;
  }

  .knoppen {
    gap: 0.4rem;
  }
}