#include <fstream>
#include <iomanip>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");
//FILE *g=fopen("aria.out","w");

int n;
double x,y,xa,ya,s,x1,y1;

int main()
{
    int i;
    f>>n;
    f>>xa>>ya;
    x1=xa;
    y1=ya;
    //g<<fixed<<setprecision(6)<<xa<<' '<<ya<<' '<<x1<<' '<<y1<<'\n';
    for(i=1;i<n;++i)
    {
        f>>x>>y;
        //fprintf(g,"%lf %lf\n",x,y);
        //g<<x<<y<<'\n';
        s=s+xa*y-ya*x;
        xa=x;
        ya=y;
    }
    s=s+x*y1-y*x1;
    //s/=2.0;
    //fprintf(g,"%lf\n",s);
    g<<fixed<<setprecision(6)<<s/2.0<<'\n';
    f.close(); g.close();
    //fclose(g);
    return 0;
}