input: Write a function to multiply two numbers without using the multiplication operator.
  assert multiply(10, 5) == 50
  assert multiply(7, 3) == 21
  assert multiply(0, 100) == 0
  assert multiply(15, 15) == 225
output: def multiply(a, b):
      result = 0
      for _ in range(abs(b)):
          result += abs(a)
      if (a < 0 and b > 0) or (a > 0 and b < 0):
          result = -result
      return result