#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;
}