#include <fstream>
#include <cmath>
std::ifstream f ("aria.in");
std::ofstream t ("aria.out");
int main()
{
    int n;
    long long p=0;
    double x,y,aux,auy,xp,yp;
    t.precision(5);
    f>>n>>aux>>auy;
    xp=aux,yp=auy;
    for (int i=1;i<n;++i){
        f>>x>>y;
        p+=x*auy-y*aux;
        aux=x,auy=y;}
    p+=xp*auy-yp*aux;
    //p=(p<0?-p:-p);
    p=abs(p);
    t<<(float)p/2;
    return 0;
}