#define _GLIBCXX_IOSTREAM 1
#include <bits/stdc++.h>

using namespace std;

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

int main() {
    int n;
    cin >> n;
    double rez{};
    vector<pair<double, double>> date(n+1);
    for (int i = 0; i < n; i++)
    {
        auto& elem = date[i];
        cin >> elem.first >> elem.second;
    }
    date[n] = date[0];

    for (int i = 0; i < n; i++)
    {
        rez += date[i].first * date[i+1].second 
            - date[i].second * date[i+1].first;
    }
    rez = rez / 2;
    cout << fixed << setprecision(5) << rez;
}