Count set bits till N

int countSetBits(int n)
    {
        n++;
        int count = n/2;
        int powerof2 = 2;

        while (powerof2 <= n) {
            int totalpairs = n/powerof2;
            count += (totalpairs/2) * powerof2; // setbits in pairs
            count += (totalpairs&1) ? (n % powerof2) : 0; // left out setbits
            powerof2 = powerof2<<1;
        }
        return count;
    }