Polygon area

input/code.cpp: In function 'void Test()':
input/code.cpp:14:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   14 |     freopen("temp\\in.txt", "r", stdin);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=10005;

LL x[N], y[N];
LL Cross(int i, int j)
{
    return x[i]*y[j] - y[i]*x[j];
}

void Test()
{
    freopen("temp\\in.txt", "r", stdin);
}
int main()
{
    // Test();
    int n;
    cin >> n;
    for(int i=1; i<=n; i++)
        cin >> x[i] >> y[i];
    LL ans = 0;
    for(int i=1; i<=n; i++)
    {
        int j = i+1;
        if(j == n+1) j = 1;
        ans += Cross(i, j);
    }
    cout << abs(ans);

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED