Polygon area

input/code.cpp: In function 'void Test()':
input/code.cpp:70:12: warning: ignoring return value of 'FILE* freopen(const char*, const char*, FILE*)' declared with attribute 'warn_unused_result' [-Wunused-result]
   70 |     freopen("temp\\in.txt", "r", stdin);
      |     ~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
input/code.cpp: In member function 'void Point::Input()':
input/code.cpp:24:14: warning: ignoring return value of 'int scanf(const char*, ...)' declared with attribute 'warn_unused_result' [-Wunused-result]
   24 |         scanf("%lf%lf", &x, &y);
      |         ~~~~~^~~~~~~~~~~~~~~~~~

#include<bits/stdc++.h>
using namespace std;
typedef long long LL;
const int N=10005;
const double EPS = 1e-8, INF=1e20, PI = acos(-1.0);

inline int Sign(const double& x)
{
    if(fabs(x) < EPS)   return 0;
    if(x < 0)           return -1;
    else                return 1;
}

struct Point
{
    double x, y;
    Point(){}
    Point(double _x, double _y)
    {
        x = _x;
        y = _y;
    }
    void Input() {
        scanf("%lf%lf", &x, &y);
    }
    bool operator == (const Point& P)const {
        return Sign(x - P.x) == 0 && Sign(y - P.y) == 0;
    }
    bool operator < (const Point& P)const {
        return Sign(x - P.x) == 0? Sign(y - P.y)<0 : x<P.x;
    }
    double operator ^ (const Point& P)const {
        return x*P.y - y*P.x;
    }
    double operator * (const Point& P)const {
        return x*P.x + y*P.y;
    }
    Point operator + (const Point& P)const {
        return Point(x+P.x, y+P.y);
    }
    Point operator * (const double& _k)const {
        return Point(x*_k, y*_k);
    }
    double len() {
        return hypot(x, y);
    }
};
struct Polygon
{
    int n;
    Point p[N];
    void Input(int _n)
    {
        n = _n;
        for(int i=0; i<n; i++)
            p[i].Input();
    }
    double Area()
    {
        double ret = 0;
        for(int i=0; i<n; i++)
            ret += (p[i] ^ p[(i+1)%n]);
        return fabs(ret)/2;
    }
};


void Test()
{
    freopen("temp\\in.txt", "r", stdin);
}
int main()
{
    // Test();
    int n;
    cin >> n;
    Polygon G;
    G.Input(n);
    cout<< (LL)(2*G.Area());

    return 0;
}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: WRONG ANSWER

Test 3

Verdict: ACCEPTED