input: Write a python function to find the highest common factor (HCF) of two numbers.
  assert find_hcf(60, 80) == 20
  assert find_hcf(12, 15) == 3
  assert find_hcf(100, 200) == 100
output: def find_hcf(a, b):
      while b:
          a, b = b, a % b
      return a