#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
const int nmax = 100005;

int main()
{
    ifstream fin ("aria.in");
    ofstream fout ("aria.out");
    ios_base::sync_with_stdio(false);
    pair<int,int> p[nmax];
    int n, i;
    double a=0;
    fin >> n;
    for(i=1; i<=n; i++)
        fin >> p[i].first >> p[i].second;
    p[n+1]=p[1];
    for(i=1; i<=n; i++)
        a=a+(p[i].first*p[i+1].second - p[i+1].first*p[i].second);
    fout << setprecision(5) << a/2;
    fin.close();
    fout.close();
    return 0;
}