#include <bits/stdc++.h>

using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
vector<pair<long double,long double> >points;
int n;
long double x,y,thepoint_x,thepoint_y;
long double calc(int point)
{
    return points[point].first*points[point+1].second-points[point].second*points[point+1].first;
}
int main()
{
    fin>>n;
    for(int i=0;i<n;i++)
    {
        fin>>x>>y;
        points.push_back({x,y});
    }
    long double arie=0;
    for(int i=0;i<n-1;i++)
        arie+=calc(i);
    arie+=points[n-1].first*points[0].second-points[n-1].second*points[0].first;
    fout<<fixed;
    fout<<setprecision(6);
    fout<<arie/2.0;
    return 0;
}