#include <fstream>
int main()
{   std::ifstream f ("aria.in");
    freopen("aria.out","w",stdout);
    int n;
    long double p=0;
    double aux,auy,xp,yp;
    f>>n>>aux>>auy;
    xp=aux,yp=auy;
    for (int i=1;i<n;++i){double x,y;
    f>>x>>y;
    p+=x*auy-y*aux;
    aux=x,auy=y;}
    p+=xp*auy-yp*aux;
    p=(p<0?-p:p);
    printf("%.5Lf",p/2);
    return 0;
}