Polygon area

#pragma GCC optimize("O3,unroll-loops")
#pragma GCC target("avx2,bmi,bmi2,lzcnt,popcnt")

#include <iostream>
#include <algorithm>
#include <vector>
#include <cmath>
#include <queue>
#include <climits>
//#include <bits/stdc++.h>
using namespace std;
typedef long long ll;

void task1(){
    ll n;
    cin >> n;

    ll a,b;
    vector<pair<ll,ll>> arr(n + 1);
    for(ll i = 0; i < n; i++){
        cin >> a >> b;
        arr[i] = {a,b};
    }
    arr[n] = arr[0];
    ll sum = 0;
    for(ll i = 0; i < n+1; i++){
        sum += (arr[i].first * arr[i+1].second) - (arr[i+1].first * arr[i].second);
       // cout << arr[i].first << " " << arr[i+1].second << endl;
       // cout << arr[i+1].first << " " << arr[i].second << endl;

    }
    cout << abs(sum) << endl;
}

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

    task1();

}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: WRONG ANSWER