#include <bits/stdc++.h>

using namespace std;

int main()
{
    freopen("aria.in", "r", stdin);
    freopen("aria.out", "w", stdout);

    int N;
    double A = 0;
    struct punct{
        double x, y;
    } aux;
    vector<punct> v;

    cin >> N;
    for(int i = 1; i <= N; i ++) {
        cin >> aux.x >> aux.y;
        v.push_back(aux);
    }

    v.push_back(v[0]);
    for(int i = 0; i < N; i ++)
        A += (v[i].x * v[i + 1].y - v[i + 1].x * v[i].y);

    cout << setprecision(5) << A / 2.0 << "\n";
    return 0;
}