这段程序会输出什么？

function pairCount(size) {
  let comparisons = 0;
  for (let left = 0; left < size; left += 1) {
    for (let right = left + 1; right < size; right += 1) {
      comparisons += 1;
    }
  }
  return comparisons;
}

console.log(pairCount(5));