#include <fstream>

using namespace std;

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

pair < double, double > p[100001];

double arie(pair < double, double > vec[], int siz) {
    double ans((vec[siz - 1].first - vec[0].first) * (vec[siz - 1].second + vec[0].second));
    for (int i = 0; i < siz - 1; ++i) {
        ans += (vec[i].first - vec[i + 1].first) * (vec[i].second + vec[i + 1].second);
    }
    return (ans / 2);
}

int main()
{
    int n;
    cin >> n;
    for (int i = 0; i < n; ++i) {
        cin >> p[i].first >> p[i].second;
    }
    cout << arie(p, n);
    return 0;
}