input: Write a function to find the least common multiple (LCM) of two numbers.
  assert lcm(6, 12) == 12
  assert lcm(4, 5) == 20
  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)