#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;

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

    int N; fin>>N;

    double aria=0;

    double xf,yf;

    double xp,yp;
    fin>>xp>>yp;
    xf=xp;
    yf=yp;

    for(int i=1;i<N;++i){
        double x,y; fin>>x>>y;

        aria+=0.5 * (xp*y - yp*x);

        xp=x; yp=y;
    }

    aria+=0.5 * (xp*yf - yp*xf);


    fout<<fixed<<setprecision(5)<<abs(aria)<<'\n';
    return 0;
}