input: Write a python function to find the least common multiple of two numbers using their greatest common denominator.
  assert lcm(12, 18) == 36
  assert lcm(5, 10) == 10
  assert lcm(7, 3) == 21
output: def lcm(a, b):
      def gcd(x, y):
          while y:
              x, y = y, x % y
          return x
      return abs(a * b) // gcd(a, b)