#include <bits/stdc++.h>

using namespace std;

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

int main() {
    int n;
    in >> n;
    long double rez{};
    vector<pair<long double, long double>> date(n+1);
    for (int i = 0; i < n; i++)
    {
        auto& elem = date[i];
        in >> 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;
    out << fixed << setprecision(5) << rez;
}