#include <iostream>
#include <fstream>
#include <vector>

using namespace std;

int n;
vector<pair<double, double>> v;

int main(int argc, const char * argv[]) {
    ifstream cin("aria.in");
    ofstream cout("aria.out");
    
    cin >> n;
    for (int i = 0; i < n; i++) {
        double x, y;
        cin >> x >> y;
        v.push_back({x, y});
    }
    
    double ans = 0;
    
    for (int i = 0; i < n; i++) {
        double aria = v[i].first * v[(i + 1) % n].second
            - v[(i + 1) % n].first * v[i].second;
        
        ans += aria;
    }
    ans /= 2;
    
    cout << ans;
    return 0;
}