body {
  background: #000;
  color: #0f0;
  font-family: monospace;
  text-align: center;
  padding: 50px;
}

input, button {
  background: #111;
  color: #0f0;
  border: 1px solid #0f0;
  padding: 10px;
  font-size: 16px;
  margin: 10px;
  width: 300px;
}

button {
  cursor: pointer;
}

.result {
  margin-top: 30px;
  font-size: 20px;
}

/* Vòng quay */
.spinner {
  margin: 30px auto;
  width: 80px;
  height: 80px;
  border: 8px solid #0f0;
  border-top: 8px solid #000;
  border-radius: 50%;
  animation: spin 1s linear infinite;
}
@keyframes spin {
  0% { transform: rotate(0deg); }
  100% { transform: rotate(360deg); }
}
.hidden {
  display: none;
}