#include <bits/stdc++.h>

using namespace std;

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

int n;
long double surface;

pair<long double, long double> coordinates[100005];

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

    coordinates[n+1] = coordinates[1];

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

    fout<<fixed<<setprecision(6)<<abs(surface)/2<<'\n';
}