input: Write a function to find the maximum sum of a subarray of size K.
  assert maximum_sum_subarray([1, 2, 3, 4, 5], 2) == 9
  assert maximum_sum_subarray([2, 1, 5, 1, 3, 2], 3) == 9
  assert maximum_sum_subarray([2, 3, 4, 1, 5], 2) == 7
output: def maximum_sum_subarray(arr, k):
      max_sum = float('-inf')
      current_sum = 0
      for i in range(len(arr)):
          current_sum += arr[i]
          if i >= k - 1:
              max_sum = max(max_sum, current_sum)
              current_sum -= arr[i - (k - 1)]
      return max_sum