Polygon area

#include <bits/stdc++.h>
using namespace std;

string int128_to_string(__int128 n) {
    bool is_negative = false;
    if (n < 0) {
        is_negative = true;
        n = -n;
    }

    string s = "";
    if (n == 0) {
        s = "0";
    } else {
        while (n > 0) {
            char digit = '0' + (n % 10);
            s += digit;
            n /= 10;
        }
        reverse(s.begin(), s.end());
    }

    if (is_negative) {
        s = "-" + s;
    }

    return s;
}

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    int n;
    cin >> n;

    vector<pair<long long, long long>> vertices(n);
    for(int i = 0; i < n; ++i){
        cin >> vertices[i].first >> vertices[i].second;
    }

    __int128 twice_area = 0;

    for(int i = 0; i < n; ++i){
        long long x_current = vertices[i].first;
        long long y_current = vertices[i].second;
        long long x_next = vertices[(i + 1) % n].first;
        long long y_next = vertices[(i + 1) % n].second;

        twice_area += (__int128)(x_current) * y_next;
        twice_area -= (__int128)(x_next) * y_current;
    }

    if(twice_area < 0){
        twice_area = -twice_area;
    }

    string result = int128_to_string(twice_area);

    cout << result;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED