#include<fstream>
#include<iomanip>
#include<cmath>
using namespace std;
int n;
double sol;
ifstream fin("aria.in");
ofstream fout("aria.out");
pair<double,double>curr,p,f;
int main()
{
    fin>>n;
    fin>>f.first>>f.second;
    p=f;
    for(int i=2;i<=n;i++)
    {
        fin>>curr.first>>curr.second;
        sol+=p.first*curr.second-p.second*curr.first;
        p=curr;
    }
    sol+=p.first*f.second-p.second*f.first;
    fout<<setprecision(5)<<fixed<<fabs(sol)/2.0;
    return 0;
}