html, body { margin: 0; }
body {
  background: #eef2f8;
  color: #1a365d;
  font: 16px/1.45 "Segoe UI", Tahoma, sans-serif;
}
main { max-width: 40rem; margin: 0 auto; padding: 3.5rem 1.15rem 4rem; }
h1 { margin: 0 0 1rem; font-weight: 650; }
textarea {
  width: 100%;
  box-sizing: border-box;
  font: 14px/1.45 ui-monospace, Consolas, monospace;
  border: 1px solid #1a365d;
  background: #fff;
  color: inherit;
  padding: 0.65rem;
  resize: vertical;
}
.row { display: flex; gap: 0.45rem; margin: 0.7rem 0; }
button {
  font: inherit;
  background: #1a365d;
  color: #fff;
  border: 0;
  padding: 0.5rem 0.95rem;
  cursor: pointer;
}
#dec { background: #fff; color: #1a365d; outline: 1px solid #1a365d; }
#err { color: #9b2c2c; }