#include <bits/stdc++.h>

using namespace std;



#define N 100005





int main()

{

    int n;

    long double area = 0;

    pair<long double, long double>a[N];

    ifstream fin("aria.in");

    ofstream fout("aria.out");

    fin >> n;

    for(int i = 0; i < n; i++){

        fin >> a[i].first >> a[i].second;

        area += (a[i - 1].first * a[i].second - a[i].first * a[i - 1].second) / 2;

    }

    area += (a[n - 1].first * a[0].second - a[0].first * a[n - 1].second) / 2;

    fout << fixed << setprecision(6);

    fout << area;

}