#include <iostream>
#include <fstream>
#include <tuple>
#include <cmath>

using namespace std;

ifstream in ("aria.in");
ofstream out ("aria.out");

int main(){

    int N;
    double arie = 0;

    pair<double, double> puncte[100005];

    in >> N;

    for(int i = 0; i < N; i++){
        in >> puncte[i].first >> puncte[i].second;
    }
    puncte[N] = puncte[0];

    for(int i = 0; i < N; i++){
        arie += puncte[i].first * puncte[i + 1].second - puncte[i + 1].first * puncte[i].second;
    }

    arie = fabs(arie) / 2.0;

    out << arie;

    return 0;
}