#include <fstream>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");

double x1,y1,x2,y2,xp,yp;

int main()
{
    int n;
    fin>>n;
    fin>>xp>>yp;
    x1=xp;
    y1=yp;
    double arie=0;
    for(int i=2;i<=n;i++)
    {
        fin>>x2>>y2;
        arie+=(x1*y2-x2*y1)/2;
        x1=x2;
        y1=y2;
    }
    arie+=(x1*yp-xp*y1)/2;
    fout<<arie;
    return 0;
}