body {
  margin: 0;
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  background: linear-gradient(135deg,slategrey,black);
  font-family: system-ui, sans-serif;
}

.box {
  text-align: center;
  background: lightgrey;
  padding: 0.5rem;
  border-radius: 1rem;
  min-height: 9rem;
}

h1 { 
  margin-top:0.5rem;
  margin-bottom: 1rem;
}

input, button {
  padding: .5rem 1rem;
  margin: .3rem 0;
  border: 1px solid white;
  border-radius: .5rem;
  font-size: 1rem;
}

button {
  background: teal;
  color: white;
  cursor: pointer;
}
button:hover { background: lightseagreen; }

#result {
  margin-top: 0.5rem;
  font-weight: 600;
}