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

double x11,y11,x22,y22,xp,yp,arie;

int main()
{
    int n;

    fin>>n;
    fin>>xp>>yp;
    x11=xp;
    y11=yp;
    for(int i=2;i<=n;i++)
    {
        fin>>x22>>y22;
        arie+=(x11*y22-x22*y11);
        x11=x22;
        y11=y22;
    }
    arie+=(x11*yp-xp*y11);
    fout<<fixed<<setprecision(6)<<arie/2;
    return 0;
}