#include <cstdio>

#include <cmath>

#include <iostream>

#include <iomanip>



using namespace std;



#define MAX 100005



struct punct {

    double x, y;

} V[MAX];



int N;



int main () {

    freopen ("aria.in", "r", stdin);

    freopen ("aria.out", "w", stdout);



    scanf ("%d", &N);

    for (int i = 0; i < N; ++i) {

        scanf ("%lf %lf", &V[i].x, &V[i].y);

    }



    V[N] = V[0];



    double answer = 0.0;



    for (int i = 0; i < N; ++i) {

        answer += (V[i].x * V[i + 1].y - V[i + 1].x * V[i].y);

    }



    cout << setprecision(6) << fixed << fabs(answer / 2.0) << '\n';



    return 0;

}