#include<iostream>
#include<fstream>
#include <math.h>
#include<iomanip>
using namespace std;
int n;
struct punct{

    double x;
    double y;

};
punct a[100005];
long double arie;
int main(){


    ifstream f("aria.in",fstream::in);
    ofstream g("aria.out",fstream::out);
    f>>n;

    for(int i=0;i<n;i++)
        f>>a[i].x>>a[i].y;
    a[n]=a[0];
    for(int i=0;i<n;i++)
        arie=arie+(a[i].x*a[i+1].y-a[i+1].x*a[i].y);

    arie=arie/2;
    g<<setprecision(6)<<fixed<<arie;

}