input: Write a python function to add two matrices of the same size.
  assert add_matrices([[1, 2], [3, 4]], [[5, 6], [7, 8]]) == [[6, 8], [10, 12]]
  assert add_matrices([[1, 1], [1, 1]], [[1, 1], [1, 1]]) == [[2, 2], [2, 2]]
  assert add_matrices([[0, 0], [0, 0]], [[0, 0], [0, 0]]) == [[0, 0], [0, 0]]
  assert add_matrices([[2, 4], [6, 8]], [[1, 3], [5, 7]]) == [[3, 7], [11, 15]]
output: def add_matrices(matrix1, matrix2):
      return [[matrix1[i][j] + matrix2[i][j] for j in range(len(matrix1[0]))] for i in range(len(matrix1))]