#include <fstream>
#include <iomanip>

const int nmax=100005;

using namespace std;

int main()
{
    ifstream fin("a.in");
    ofstream fout("a.out");

    int n,i;
    long double x[nmax],y[nmax];

    fin>>n;
    for(i=1;i<=n;i++) {
        fin>>x[i]>>y[i];
    }

    x[n+1]=x[1];
    y[n+1]=y[1];

    long double s=0;
    for(i=1;i<=n;i++)
        s+=x[i]*y[i+1]-x[i+1]*y[i];
    fout<<setprecision(6)<<fixed<<s/2.0;

    return 0;

}