#include <bits/stdc++.h>


using namespace std;

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

const int Nmax = 1e5 + 5;

pair < long double , long double > a[Nmax + 5];

long double ar;

int n;

int main()
{
    fin >> n;
    for(int i = 1 ; i <= n ; i++)
        fin >> a[i].first >> a[i].second;
    a[n + 1] = a[1];
    n++;
    for(int i = 1 ; i <= n ; i++)
        ar += a[i].first * a[i + 1].second - a[i + 1].first * a[i].second;
    fout << fixed << setprecision(5) << (long double) ar / 2 << "\n";
    fin.close();
    fout.close();
}