input: Write a python function to add two complex numbers.
  assert add_complex_numbers(complex(1, 2), complex(3, 4)) == complex(4, 6)
  assert add_complex_numbers(complex(-1, -1), complex(1, 1)) == complex(0, 0)
  assert add_complex_numbers(complex(0, 0), complex(0, 0)) == complex(0, 0)
output: def add_complex_numbers(c1, c2):
      return c1 + c2