input: Write a python function to sum two integers.
  assert sum_two_ints(1, 2) == 3
  assert sum_two_ints(-1, 1) == 0
  assert sum_two_ints(0, 0) == 0
  assert sum_two_ints(123, 456) == 579
output: def sum_two_ints(a, b):
      return a + b