#include <bits/stdc++.h>

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

int n;
double sol;
pair<double, double> points[100005];

int main()
{

    fin >> n;

    for(int i = 1; i <= n; ++i) fin >> points[i].first >> points[i].second;
    points[n + 1] = points[1];

    for(int i = 1; i <= n; ++i){
        sol = sol + (points[i + 1].second * points[i].first - points[i + 1].first * points[i].second);
    }

    fout << fixed << setprecision(6) << sol / 2.0;

    return 0;
}