Формулы

/***
Геомтерия
***/
#include <bits/stdc++.h>

using namespace std;

double dist(double x, double y, double x2, double y2) {
    double d = sqrt((x - x2) * (x - x2) + (y - y2) * (y - y2));
    return d;
}

double area(double x, double y, double x2, double y2, double x3, double y3) {
    double a = dist(x, y, x2, y2);
    double b = dist(x, y, x3, y3);
    double c = dist(x2, y2, x3, y3);
    double u = (a + b + c) / 2;
    double S = sqrt(u * (u - a) * (u - b) * (u - c));
    return S;
}

int main(){
    int n;
    cin >> n;

    int x[n + 1], y[n + 1];
    for (int i = 1; i <= n; i++) {
        cin >> x[i] >> y[i];
    }

    double s1 = 0, s2 = 0;
    for (int i = 1; i < n; i++) {
        s1 += x[i] * y[i + 1];
        s2 += y[i] * x[i + 1];
    }

    s1 += x[n] * y[1];
    s2 += y[n] * x[1];

    double S = fabs(s1 - s2) / 2;
}