Beautiful Cuple

you have a sequence of n integers. A pair of numbers x and y are considered to be k-beautiful if their binary representation differs from each other in exactly k bits. For example, if k  = 2 , then the pair of numbers x  = 5 and y  = 3 is k-interesting , since the binary representations x = 101 and y = 011 differ in exactly two bits. find how many pairs of indices ( i , j ) exist in his sequence