#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

int main()
{
    int n;
    fin>>n;
    long double x1,y1;
    fin>>x1>>y1;
    long double x0=x1,y0=y1;
    long double arie=0;
    for (int i=2; i<=n; i++)
    {
        long double x2,y2;
        fin>>x2>>y2;
        arie += (x1 * y2 - x2 * y1) / 2.0;
        if (i==n)
            arie=arie+(x2 * y0 - x0 * y2)/2.0;
        x1=x2;
        y1=y2;
    }
    fout<<setprecision(10)<<fixed<< arie;
    return 0;
}