# include <fstream>
# include <iomanip>
using namespace std;
ifstream fin ("aria.in");
ofstream fout ("aria.out");
double x[100001], y[100001];
int main()
{
    int n, i;
    double sol=0;
    fin>>n;
    for (i=0; i<n; i++)
        fin>>x[i]>>y[i];
    x[n]=x[0];
    y[n]=y[0];
    for (i=0; i<n+1; i++)
       sol+=(x[i]*y[i+1]-x[i+1]*y[i]);
    sol/=2;
    fout<<fixed<<setprecision(5)<<sol;
    return 0;
}