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

int main() 
{
    ifstream cin("aria.in");
    ofstream cout("aria.out");
	int n; cin >> n;
    vector<pair<double, double>> v;
    v.push_back({0, 0});
    double x, y; 
    for (int i = 1; i <= n; i ++)
    {
        cin >> x >> y;
        v.push_back({x, y});
    }
    v[0] = {x, y};
    double ans = 0;
    for (int i = 1; i <= n; i ++)
    {

        ans += abs(v[i].second * v[i - 1].first) + abs(v[i].first * v[i - 1].second);
    }
    ans /= 2;
    ans = abs(ans);
    cout << ans;
}