#include <bits/stdc++.h>

using namespace std;
ifstream in("aria.in");
ofstream out("aria.out");

const int NMAX = 100005;
pair< double, double> v[NMAX];

int main() {
    int n;
    in >> n;
    double ans = 0;
    for(int i = 1; i <= n; i ++)
        in >> v[i].first >> v[i].second;
    v[n + 1] = v[1];
    for(int i = 1; i <= n; i ++)
        ans += (v[i].first * v[i + 1].second - v[i + 1].first * v[i].second);
    out << fixed << setprecision(6) << ans / 2;

    return 0;
}