#include <bits/stdc++.h>
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
const int Nmax = 100005;
struct P
{
   double ox , oy;
};
P a[Nmax];
int n;
int main()
{
    double ar = 0;
    fin >> n;
    for(int i = 1 ; i <= n ; i++)
        fin >> a[i] . ox >> a[i] . oy;
    a[0] = a[n];
    a[n + 1] = a[1];
    for(int i = 1 ; i <= n ; i++)
        ar += a[i] . ox * (a[i + 1] . oy - a[i - 1] . oy);
    fout << setprecision(5) << fixed << ar / 2  << "\n";
    fin.close();
    fout.close();
    return 0;
}