#include<fstream>
#include<cmath>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

struct QQ{
    double x;
    double y;
};


int main(){
    int n,i;
    long double space=0;
    QQ x1, x2, x3;

    f>>n;
    if(n>2){
        for(i=1; i<=n; ++i){
            if(i==1){
                f>>x1.x>>x1.y;
                x3=x1;
            }
            else{
                f>>x2.x>>x2.y;
                    space+=x1.x*x2.y - x2.x*x1.y;

                x1=x2;
            }
        }

        x2=x3;
        space+=x1.x*x2.y - x2.x*x1.y;
    }


    g<<fixed;
    g.precision(5);
    g<<space/2;

return 0;
}