#include <fstream>
//#include <iostream>
#include <iomanip>
using namespace std;
ifstream cin("aria.in");
ofstream cout("aria.out");
int n;
float ux, uy, x, y, x0, y0;
float a=0;
int main()
{
    cin>>n;
    cin>>ux>>uy;
    x0=ux; y0=uy;
    for(int i=2;i<=n;i++)
    {
        cin>>x>>y;
        a=(float)a+(ux*y-x*uy)/2;
        ux=x; uy=y;
    }
    a=(float)a+(x*y0-x0*y)/2;
    cout<<fixed<<setprecision(5)<<a;
    return 0;
}