input: Write a python function to calculate the sum of all multiples of 3 or 5 below a given number.
  assert sum_multiples(1000) == 233168
  assert sum_multiples(10) == 23
  assert sum_multiples(20) == 78
output: def sum_multiples(n):
      return sum(i for i in range(n) if i % 3 == 0 or i % 5 == 0)