Polygon area

input/code.cpp: In function 'int main()':
input/code.cpp:15:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   15 |     freopen("input.txt", "r", stdin);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~

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

typedef long long C;
typedef complex<C> P;
#define X real()
#define Y imag()

C cross(const P& a, const P& b) {
    return (conj(a)*b).Y;
}

int main () {

    freopen("input.txt", "r", stdin);
    int n;
    cin >> n;

    long long A = 0;

    int x0, y0;
    cin >> x0 >> y0;
    P prev = {x0, y0};
    while(--n) {
        int x, y;
        cin >> x >> y;
        P cur = {x, y};
        A += cross(prev, cur);
        prev = cur;
    }
    A += cross(prev, {x0, y0});

    cout << A << '\n';
    
    return 0;
}

Test 1

Verdict: WRONG ANSWER

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: WRONG ANSWER