How is it possible to get given output with code below ??

#include <iostream>
#include <vector>
#include <future>
#include <algorithm>
#include <cmath>

using namespace std;

void solve() {
    int n = 1000000000;
    long long ans = n;
    ans = (ans * (ans + 1)) / 2;
    long i{0};

    while(true) {
        const auto num = pow(2, i++);
        if(num > n) {
            break;
        }
        cout << ans << " - " << (2 * num);
        ans -= 2 * num;
        cout << " = "<< ans << endl;
        if(i > 10){
            break;
        }
    }
}


int main() {
    solve();
    return 0;
}

Getting the output as:

500000000500000000 - 2 = 500000000500000000
500000000500000000 - 4 = 500000000500000000
500000000500000000 - 8 = 500000000500000000
500000000500000000 - 16 = 500000000500000000
500000000500000000 - 32 = 500000000500000000
500000000500000000 - 64 = 500000000499999936
500000000499999936 - 128 = 500000000499999808
500000000499999808 - 256 = 500000000499999552
500000000499999552 - 512 = 500000000499999040
500000000499999040 - 1024 = 500000000499998016
500000000499998016 - 2048 = 500000000499995968

I tried to subtract the required value from ans using the logic below

int d = 1;
     while(d <= n) {
     cout << res << " - " << d << endl;
     res -= 2 * d;
     d *= 2;
 }

Where res have same value as ans in code snipped above, where am I lacking?? Please help