#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

#define Nmax 100005

struct punct{

    double x;
    double y;

} V[Nmax];

int N;
double S;

void citire(){

    ifstream f("aria.in");

    f >> N;

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

    f.close();
}

void rezolva(){

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

    for ( int i = 1; i <= N; ++i )
        S += ( V[i].x * V[i + 1].y - V[i + 1].x * V[i].y );

    S /= 2.0;
}

void afis(){

    ofstream g("aria.out");

    g.precision(10);
    g << S << "\n";

    g.close();
}

int main(){

    citire();
    rezolva();
    afis();

    return 0;
}