#include <fstream>
using namespace std;

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

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

    pair <double , double> coordonate[100001];
    for (int indice = 1 ; indice <= numar_puncte ; indice++)
        cin >> coordonate[indice].first >> coordonate[indice].second;

    double arie = coordonate[numar_puncte].first * coordonate[1].second - coordonate[1].first * coordonate[numar_puncte].second;
    for (int indice = 1 ; indice < numar_puncte ; indice++)
        arie += coordonate[indice].first * coordonate[indice + 1].second - coordonate[indice + 1].first * coordonate[indice].second;

    cout << fixed << (arie /= 2.0);
    cout.close(); cin.close();
    return 0;
}