#include <iomanip>
#include <fstream>

using namespace std;

ifstream f("aria.in");
ofstream g("aria.out");

const int Nmax = 100005;
int n;
double aria;
pair <double, double> a[Nmax];

int main()
{
    f>>n;
    for(int i = 0; i < n; i++) f>>a[i].first>>a[i].second;
    a[n] = a[0];
    for(int i = 0; i < n; i++)
    {
        aria = aria + a[i].first*a[i+1].second - a[i].second*a[i+1].first;
    }
    aria /= 2;
    g<<fixed<<setprecision(6)<<aria<<'\n';
    return 0;
}