solution.js

const S = "SMS messages are really short";
const K = 12;

function solution(S, K) {
  const arr = S.split(" ").map(word => word.length);

  let messagesCount = 1;
  let message = arr[0];

  for (let i = 1; i < arr.length; i += 1) {
    if (arr[0] > K || arr[i] > K) return -1;

    if (message + 1 + arr[i] <= K) {
      message += 1 + arr[i];
    } else {
      messagesCount += 1;
      message = arr[i];
    }
  }

  return messagesCount;
}

solution(S, K);