さすがにこのへんはまだ簡単か。

提出 #4270420 - AtCoder Beginners Selection https://atcoder.jp/contests/abs/submissions/4270420?lang=ja

function Main(input) {
  const matches = input.match(/1/g)
  console.log(matches ? matches.length : 0)
}
Main(require('fs').readFileSync('/dev/stdin', 'utf8'))