/* ===== Reset & Base ===== */
*, *::before, *::after {
  box-sizing: border-box;
  margin: 0;
  padding: 0;
}

body {
  font-family: system-ui, -apple-system, sans-serif;
  font-size: 1rem;
  line-height: 1.5;
  color: #1a1a1a;
  background-color: #f0f2f5;
  min-height: 100vh;
  display: flex;
  align-items: flex-start;
  justify-content: center;
  padding: 1rem;
}

/* ===== Main container ===== */
main {
  background: #ffffff;
  border-radius: 8px;
  box-shadow: 0 2px 8px rgba(0, 0, 0, 0.12);
  padding: 1.5rem;
  width: 100%;
  max-width: 480px;
  display: flex;
  flex-direction: column;
  gap: 1.5rem;
}

/* ===== Typography hierarchy ===== */
h1 {
  font-size: 1.75rem;
  font-weight: 700;
  color: #1a1a1a;
  text-align: center;
}

h2 {
  font-size: 1.125rem;
  font-weight: 600;
  color: #1a1a1a;
  margin-bottom: 0.5rem;
}

/* ===== Form ===== */
#calculator-form {
  display: flex;
  flex-direction: column;
  gap: 1.25rem;
}

fieldset {
  border: 1px solid #d0d0d0;
  border-radius: 6px;
  padding: 1rem;
}

legend {
  font-size: 0.9375rem;
  font-weight: 600;
  color: #1a1a1a;
  padding: 0 0.5rem;
}

fieldset > div + div {
  margin-top: 1rem;
}

/* ===== Labels & Inputs ===== */
label {
  display: block;
  font-size: 0.9375rem;
  font-weight: 500;
  color: #1a1a1a;
  margin-bottom: 0.375rem;
}

input[type="number"] {
  display: block;
  width: 100%;
  min-height: 44px;
  padding: 0.625rem 0.75rem;
  border: 2px solid #767676;
  border-radius: 4px;
  font-size: 1rem;
  color: #1a1a1a;
  background: #ffffff;
  -moz-appearance: textfield;
  appearance: textfield;
}

input[type="number"]::-webkit-inner-spin-button,
input[type="number"]::-webkit-outer-spin-button {
  -webkit-appearance: none;
}

input[type="number"]:focus {
  outline: 3px solid #1565c0;
  outline-offset: 1px;
  border-color: #1565c0;
}

/* ===== Operation buttons ===== */
[role="group"] {
  display: grid;
  grid-template-columns: 1fr 1fr;
  gap: 0.5rem;
}

button[name="operation"] {
  min-height: 44px;
  min-width: 44px;
  padding: 0.625rem 0.5rem;
  border: 2px solid #767676;
  border-radius: 4px;
  background: #f5f5f5;
  color: #1a1a1a;
  font-size: 0.9375rem;
  font-weight: 500;
  cursor: pointer;
  transition: background-color 0.15s, border-color 0.15s, color 0.15s;
}

button[name="operation"]:hover {
  background: #e0e0e0;
  border-color: #1565c0;
}

button[name="operation"][aria-pressed="true"] {
  background: #1565c0;
  border-color: #1565c0;
  color: #ffffff;
}

button[name="operation"]:focus-visible {
  outline: 3px solid #1565c0;
  outline-offset: 2px;
}

/* ===== Action buttons (Calculate / Clear) ===== */
#calculator-form > div {
  display: flex;
  gap: 0.75rem;
}

#calculate-btn,
#clear-btn {
  flex: 1;
  min-height: 44px;
  min-width: 44px;
  padding: 0.625rem 1rem;
  border: 2px solid transparent;
  border-radius: 4px;
  font-size: 1rem;
  font-weight: 600;
  cursor: pointer;
  transition: background-color 0.15s;
}

#calculate-btn {
  background: #1565c0;
  color: #ffffff;
}

#calculate-btn:hover {
  background: #0d47a1;
}

#clear-btn {
  background: #f5f5f5;
  color: #1a1a1a;
  border-color: #767676;
}

#clear-btn:hover {
  background: #e0e0e0;
}

#calculate-btn:focus-visible,
#clear-btn:focus-visible {
  outline: 3px solid #1565c0;
  outline-offset: 2px;
}

/* ===== Result section ===== */
section:first-of-type {
  border: 1px solid #d0d0d0;
  border-radius: 6px;
  padding: 1rem;
  background: #f8f9fa;
}

#result {
  display: block;
  font-size: 1.5rem;
  font-weight: 700;
  color: #1a1a1a;
  min-height: 2rem;
}

/* ===== Error section ===== */
#error-message {
  font-size: 0.9375rem;
  font-weight: 500;
  color: #c62828;
  min-height: 1.25rem;
}

/* ===== Tablet (768px+) ===== */
@media (min-width: 768px) {
  body {
    padding: 2rem;
    align-items: center;
  }

  main {
    padding: 2rem;
    gap: 2rem;
  }

  h1 {
    font-size: 2rem;
  }

  fieldset > div {
    display: flex;
    align-items: center;
    gap: 1rem;
  }

  fieldset > div + div {
    margin-top: 0.75rem;
  }

  fieldset > div label {
    flex: 0 0 140px;
    margin-bottom: 0;
  }

  fieldset > div input {
    flex: 1;
  }

  [role="group"] {
    grid-template-columns: repeat(4, 1fr);
  }
}

/* ===== Desktop (1024px+) ===== */
@media (min-width: 1024px) {
  main {
    max-width: 560px;
  }

  h1 {
    font-size: 2.25rem;
  }

  #result {
    font-size: 1.75rem;
  }
}