#include <bits/stdc++.h>

using namespace std;

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

int n;
pair<long double, long double> coord[100005];
long double area;

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0); cout.tie(0);

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

    coord[n+1] = coord[1];

    for ( int i = 1; i <= n; ++i )
        area += (coord[i].first*coord[i+1].second-coord[i+1].first*coord[i].second);

    fout<<fixed<<setprecision(12)<<(long double)1/2*area<<'\n';
}