#include <fstream>
#include <iomanip>

#define DL long double

using namespace std;

const int Nmax = 100005;

struct punct{
    DL x, y;
};

punct v[Nmax];

int main(){
    ifstream fin("aria.in");
    ofstream fout("aria.out");

    int n;
    DL area;

    fin >> n;

    for(int i = 1; i <= n; i++){
        fin >> v[i].x >> v[i].y;
    }

    v[0] = v[n];
    v[n + 1] = v[1];

    area = 0;
    for(int i = 1; i <= n; i++){
        area += (v[i].x * v[i + 1].y - v[i].y * v[i + 1].x);
    }
    area = 0.5 * area;

    fout << setprecision(5) << area;

    return 0;
}