input: Write a function to perform matrix multiplication.
  assert matrix_multi([[1, 2], [3, 4]], [[2, 0], [1, 2]]) == [[4, 4], [10, 8]]
  assert matrix_multi([[1, 0, 2], [-1, 3, 1]], [[3, 1], [2, 1], [1, 0]]) == [[5, 1], [4, 2]]
  assert matrix_multi([[2, 4], [3, 4]], [[1, 2], [1, 3]]) == [[6, 16], [7, 18]]
output: def matrix_multi(A, B):
      result = [[0 for _ in range(len(B[0]))] for _ in range(len(A))]
      
      for i in range(len(A)):
          for j in range(len(B[0])):
              for k in range(len(B)):
                  result[i][j] += A[i][k] * B[k][j]
      
      return result