input: Write a python function to find the maximum sum of any contiguous subarray of size `k`.
  assert max_sum_subarray([1, 2, 3, 4, 5], 3) == 12
  assert max_sum_subarray([2, 1, 5, 1, 3, 2], 3) == 9
  assert max_sum_subarray([2, 3, 4, 1, 5], 2) == 8
output: def max_sum_subarray(arr, k):
      if len(arr) < k:
          return 0
  
      max_sum = current_sum = sum(arr[:k])
  
      for i in range(k, len(arr)):
          current_sum += arr[i] - arr[i - k]
          max_sum = max(max_sum, current_sum)
  
      return max_sum