#include <fstream>
#include <iomanip>

using namespace std;

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

int n, i;

double ARIA;

struct punct {
    double x;
    double y;
}v[100010];

double arie (double x1,double y1, double x2, double y2, double x3, double y3) {

    double a=(x2-x1)*(y3-y1)-(x3-x1)*(y2-y1);
    return a;
}

int main () {

    fin>>n;

    for (i=1;i<=n;i++)
        fin>>v[i].x>>v[i].y;
    for (i=2;i<n;i++)
        ARIA+=arie(0,0,v[i-1].x,v[i-1].y,v[i].x,v[i].y);

    fout << setprecision(6) << fixed;
    fout<<ARIA<<"\n";

    return 0;
}