#include <iostream>
#include <fstream>
#include <iomanip>

using namespace std;

int main()
{
    ifstream fin ("aria.in");
    ofstream fout ("aria.out");
    int n;
    double xinit,yinit,xant,yant,x,y,arie=0;
    fin>>n>>x>>y;
    xinit=xant=x;
    yinit=yant=y;
    for (int i=2;i<=n;++i)
    {
        fin>>x>>y;
        arie+=xant*y-x*yant;
        xant=x;
        yant=y;
    }
    arie+=xant*yinit-xinit*yant;
    arie*=0.5;
    fout<<setprecision(5)<<fixed<<arie;
    return 0;
}