#include <bits/stdc++.h>

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

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

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

    return 0;
}