#include<fstream>

#include<iomanip>

#include<cmath>



using namespace std;



ifstream fin("aria.in");

ofstream fout("aria.out");



const int Nmax = 100008;



struct Point {double x, y;} V[Nmax];



int N; long double Arie = 0;



double A(const Point &P1, const Point&P2){



    return P1.x * P2.y - P1.y * P2.x;

}



int main(){



    fin >> N; for(int i = 1; i <= N; ++i) fin >> V[i].x >> V[i].y;



    V[N + 1] = V[1];



    for(int i = 1; i <= N; i++) Arie += A(V[i], V[i + 1]);



    fout <<fixed << setprecision(6) <<  fabs(Arie / 2.0);



    return 0;

}