input: Write a function to calculate the sum of all multiples of 3 or 5 below a given number.
  assert sum_multiples(10) == 23
  assert sum_multiples(20) == 78
  assert sum_multiples(50) == 543
output: def sum_multiples(n):
      return sum(x for x in range(n) if x % 3 == 0 or x % 5 == 0)