input: Write a function to find the greatest common divisor (GCD) of two numbers.
  assert find_gcd(12, 18) == 6
  assert find_gcd(100, 75) == 25
  assert find_gcd(37, 600) == 1
output: def find_gcd(a, b):
      while b:
          a, b = b, a % b
      return a