#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;

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

const int nmax = 1e5+5;
pair<double,double> p[nmax];

int main()
{
    ios_base::sync_with_stdio(false);
    int n, i;
    double area=0;
    fin >> n;
    for(i=1; i<=n; i++)
        fin >> p[i].first >> p[i].second;

    p[n+1]=p[1], p[0]=p[n];
    for(i=1; i<=n; i++)
        area+=p[i].first*(p[i+1].second-p[i-1].second);
    fout << setprecision(5) << area/2.0;
    fin.close();
    fout.close();
    return 0;
}