#include <bits/stdc++.h>
using namespace std;

int n;
pair <double, double> a[100005];
double ans;

int main(){
    ifstream cin ("aria.in");
    ofstream cout ("aria.out");
    cin >> n;
    for (int i=1; i<=n; i++) cin >> a[i].first >> a[i].second;
    a[n+1] = a[1];
    for (int i=1; i<=n; i++){
        ans += (a[i].first * a[i+1].second - a[i+1].first * a[i].second);
    }
    cout << fixed << setprecision(5) << ans/2;
    return 0;
}