#include <fstream>
#include <iomanip>
using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");
double modul(double val){
    if(val < 0)
        return -val;
    return val;
}
int main()
{
    int n;
    double sol = 0, xi, yi, xant, yant, x, y;
    in>>n>>x>>y;
    xi = xant = x;
    yi = yant = y;
    for(int i=2;i<=n;i++){
        in>>x>>y;
        sol += (xant * y - yant * x);
        xant = x;
        yant = y;
    }
    in.close();
    sol += (x * yi - y * xi);
    sol = modul(sol)/2;
    out<<fixed<<setprecision(5)<<sol<<"\n";
    out.close();
    return 0;
}