Weird Unsigned substraction

#include <bits/stdc++.h>
using namespace std;
#define d(x)                cout << #x << " = " << (x) << endl;

int main(void) {
    #ifndef ONLINE_JUDGE
        clock_t tStart = clock();
        freopen("in.txt", "r", stdin);
        freopen("out.txt", "w", stdout);
    #endif

    int n = 1000000000;
    unsigned long long int nsum = (n*(n+1))/2;
    cout << nsum << endl;

    unsigned long long int p = 1000000000;
    unsigned long long int psum = (p*(p+1))/2;
    cout << psum << endl;

    return 0;
}