input: Write a python function to convert a string to an integer and an integer to a string.
  assert str_to_int("123") == 123
  assert str_to_int("456") == 456
  assert int_to_str(123) == "123"
  assert int_to_str(456) == "456"
output: def str_to_int(s):
      return int(s)
  
  def int_to_str(i):
      return str(i)