#include<iostream>
#include<fstream>
#include <math.h>
#include<iomanip>

using namespace std;

int N;
struct punct{

    double x;
    double y;

};
punct v[100005];
double S;

int main(){


    ifstream fin("aria.in",fstream::in);
    ofstream fout("aria.out",fstream::out);
    fin>>N;

    for(int i=0;i<N;i++)
        fin>>v[i].x>>v[i].y;
    v[N]=v[0];
    for(int i=0;i<N;i++)
        S=S+(v[i].x*v[i+1].y-v[i+1].x*v[i].y);

    S=S/2;
    fout<<setprecision(6)<<fixed<<S;

}