If number formed by last four digits is divisible by 16.
int last = str [ n - 1 ] - '0' ; int second_last = str [ n - 2 ] - '0' ; int third_last = str [ n - 3 ] - '0' ; int fourth_last = str [ n - 4 ] - '0' ; return ( ( fourth_last * 1000 + third_last * 100 + second_last * 10 + last ) % 16 == 0 ) ; }