input: Write a function to find the least common multiple of two integers.
  assert lcm(15, 20) == 60
  assert lcm(7, 5) == 35
  assert lcm(21, 6) == 42
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)