Write a program that prints the maximum element in the area below the main diagonal including the diagonal of the square matrix.

def max_under_diagonal(matrix):
  res = 0
  for i in range(len(matrix) - 1): # no need to check the last row
    for j in range(i + 1):
      if matrix[i][j] > res:
        res = matrix[i][j]
  return res

max_under_diagonal([[1, 2, 3, 4],
                    [5, 6, 7, 8],
                    [9, 10, 11, 12],
                    [13, 14, 15, 16]]) # 15