#include <fstream>
using namespace std;

ifstream cin ("aria.in");
ofstream cout ("aria.out");

int main ()
{
    int numar_puncte;
    cin >> numar_puncte;

    long double arie = 0;
    pair <long double , long double> coordonate[2] , inceput;
    cin >> coordonate[0].first >> coordonate[0].second; inceput = coordonate[0];
    for (int indice = 2 ; indice <= numar_puncte ; indice++)
    {
        cin >> coordonate[1].first >> coordonate[1].second;
        arie += coordonate[0].first * coordonate[1].second - coordonate[1].first * coordonate[0].second;
        swap(coordonate[0] , coordonate[1]);
    }

    cout << fixed << ((arie += coordonate[0].first * inceput.second - inceput.first * coordonate[0].second) /= 2.0);
    cout.close(); cin.close();
    return 0;
}