input: Write a python function to determine if a number is odd or even.
  assert odd_or_even(1) == "odd"
  assert odd_or_even(2) == "even"
  assert odd_or_even(15) == "odd"
  assert odd_or_even(20) == "even"
output: def odd_or_even(n):
      if n % 2 == 0:
          return "even"
      else:
          return "odd"