input: Write a python function to add two numbers.
  assert add_two_numbers(5, 5) == 10
  assert add_two_numbers(3, 7) == 10
  assert add_two_numbers(0, 0) == 0
  assert add_two_numbers(-1, 1) == 0
output: def add_two_numbers(a, b):
      return a + b