input: Write a Python function to determine the number of zero bits in the binary representation of a given integer.  
  assert count_unset_bits(2) == 1  
  assert count_unset_bits(4) == 2  
  assert count_unset_bits(6) == 1
output: def count_unset_bits(n):
      return bin(n).count('0') - 1 if n != 0 else 1