input: Write a python function to count unset bits of a given number.
  assert count_unset_bits(2) == 1
  assert count_unset_bits(4) == 2
  assert count_unset_bits(6) == 1
  
output: def count_unset_bits(n):
      count = 0
      while n > 0:
          if n & 1 == 0:
              count += 1
          n >>= 1
      return count