const max_sub_array_of_size_k = function(k, arr) {
  // The time complexity of the above algorithm will be O(N*K)
  let maxSum = 0;
  let windowSum = 0;
  for (let i = 0, len = arr.length; i < len-k; i++) {
      windowSum = 0;
      for (let j = i; j < i + k; j++) {
        windowSum += arr[j]
      }
      maxSum = Math.max(maxSum, windowSum);
  }
  return maxSum;
};


console.log(`Maximum sum of a subarray of size K: ${max_sub_array_of_size_k(3, [2, 1, 5, 1, 3, 2])}`)
console.log(`Maximum sum of a subarray of size K: ${max_sub_array_of_size_k(2, [2, 3, 4, 1, 5])}`)