#include <iostream>
#include <fstream>

using namespace std;

const int NMax = 100005;

struct pct
{
    double x,y;
}v[NMax];

double Arie(pct a, pct b)
{
    return a.x * b.y - b.x * a.y;
}


int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");

    int n;
    fin >> n;

    for(int i=1; i<=n; ++i)
    {
        fin >> v[i].x >> v[i].y;
    }

    v[n+1].x = v[1].x;
    v[n+1].y = v[1].y;

    double arie = 0;
    for(int i=1; i<=n; ++i)
        arie += (double)Arie(v[i], v[i+1]);

    fout << (double)arie/2;


    return 0;
}