Write a python function to count unset bits of a given number.
def count_unset_bits(n): 
    count = 0
    x = 1
    while(x < n + 1): 
        if ((x & n) == 0): 
            count += 1
        x = x << 1
    return count  
<s>[INST]<<SYS>>Given a task and its code, generate test cases for the code.<</SYS>>Write a python function to count unset bits of a given number.
def count_unset_bits(n): 
    count = 0
    x = 1
    while(x < n + 1): 
        if ((x & n) == 0): 
            count += 1
        x = x << 1
    return count  [/INST]assert count_unset_bits(2) == 1
assert count_unset_bits(4) == 2
assert count_unset_bits(6) == 1</s>