function toNumber(value) {
  const n = Number(value);
  return Number.isFinite(n) ? n : 0;
}

function updateSum() {
  const aEl = document.getElementById("numA");
  const bEl = document.getElementById("numB");
  const resultEl = document.getElementById("result");

  if (!aEl || !bEl || !resultEl) return;

  const sum = toNumber(aEl.value) + toNumber(bEl.value);
  resultEl.value = String(sum);
}

document.addEventListener("input", (e) => {
  const target = e.target;
  if (!(target instanceof HTMLElement)) return;
  if (target.id !== "numA" && target.id !== "numB") return;
  updateSum();
});

document.addEventListener("DOMContentLoaded", () => {
  updateSum();
});