#include <iostream>
#include <fstream>
#include <iomanip>
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
#define MAX 100010

pair<double, double> a[MAX];

int main()
{
    int n, i;
    double arie = 0;
    fin >> n;
    for(i = 1 ; i <= n ; i++)
    {
        fin >> a[i].first >> a[i].second;
    }
    a[n + 1] = a[1];
    for(i = 1 ; i <= n ; i++)
    {
        arie += 1ll * (a[i].first - a[i + 1].first) * (a[i].second + a[i + 1].second);
    }
    arie = max(arie, -arie);
    fout << fixed << setprecision(5) << (double) arie / 2.0 << '\n';
}