#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

int main()
{
    ifstream f("aria.in");
    ofstream g("aria.out");
    int n;
    double rez=0;
    f>>n;
    double x1=0,x2=0,y1=0,y2=0,xp=0,yp=0;
    f>>x1>>y1;
    xp=x1;yp=y1;
    for(int i=1;i<n;i++)
    {
        f>>x2>>y2;
        rez+=x1*y2-x2*y1;
        x1=x2;
        y1=y2;
    }
    rez+=xp*y2-x2*yp;
    g<<setprecision(5)<<rez;
    return 0;
}