#include <fstream>
#include <iomanip>

const int nmax=100001;

using namespace std;

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

    int n,i;
    double x0,y0,x1,y1,x2,y2;

    fin>>n;
    fin>>x0>>y0;

    x2=x0;
    y2=y0;
    double s=0;
    for(i=1;i<n;i++) {
        x1=x2;
        y1=y2;
        fin>>x2>>y2;
        s+=x2*y1-x1*y2;
    }
    s+=x0*y2-x2*y0;
    if(s<0)
        s=-s;
    s/=2;

    fout<<setprecision(10)<<s<<fixed;

    fin.close();
    fout.close();

    return 0;
}