#include <fstream>
#include<iomanip>
#define tip long double
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
tip xi,yi,xs,ys,xd,yd,aria;
int n;
int main()
{
    f>>n>>xi>>yi;
    xs=xi;ys=yi;
    for(int i=2;i<=n;i++)
    {
        f>>xd>>yd;
        aria+=(xs-xd)*(ys+yd);
        xs=xd;ys=yd;
    }
    xd=xi;yd=yi;
    aria+=(xs-xd)*(ys+yd);
    aria/=2.0;
    g<<fixed<<setprecision(10)<<aria;
    return 0;
}