Polygon area

#include <bits/stdc++.h>

using namespace std;

//Definitions for quicker writing
#define REP(i,a,b) for (int i = a; i < b; i++)
#define clz __builtin_clz
#define ctz __builtin_ctz
#define popct __builtin_popcount
#define PB push_back
#define MP make_pair
#define F first
#define S second
#define X real()
#define Y imag()

//Typedefs for quicker writing
typedef long long ll;
typedef vector<int> vi;
typedef vector<long long> vl;
typedef pair<int,int> pi;
typedef pair<long long, long long> pl;
typedef complex<long long> po;

//Max values
const long long lmx = LLONG_MAX;
const int imx = INT_MAX;

vector<po> points(1001);

ll cross(po v, po w){
    return v.X * w.Y - v.Y * w.X;
}

ll shoelace(ll n){
    ll sum = 0;
    REP(i,0,n-1){
        sum += cross(points[i],points[i+1]);
    }
    sum += cross(points[n-1],points[0]);
    return abs(sum);
}

int main() {
	//IO optimization
	ios::sync_with_stdio(0);
	cin.tie(0);
	
	//Input definition
	ll n;

	//Read In
	cin >> n;

    ll a,b;
    REP(i,0,n){
        cin >> a >> b;
        points[i] = {a,b};
    }

	//Write out
	cout << shoelace(n) << "\n";
		
	//Return
	return 0;

}

Test 1

Verdict: ACCEPTED

Test 2

Verdict: ACCEPTED

Test 3

Verdict: ACCEPTED