#include <fstream>
#include <algorithm>
#include <iomanip>
using namespace std;
ifstream in ("aria.in");
ofstream out ("aria.out");
int n; long long s;
pair <int, int> a[1000001];
int main()
{
    in >> n;
    for (int i = 1; i <= n; i++)
        in >> a[i].first >> a[i].second;
    a[n+1].first = a[1].first; a[n+1].second = a[1].second;
    for (int i = 1; i <= n; i++)
        s = s + a[i].first * a[i+1].second - a[i+1].first * a[i].second;
    out << fixed << setprecision(6) << fabs(s/2) << '\n';
    out.close(); return 0;
}