#include <fstream>
#define Punct pair <int,int>
#define x first
#define y second
using namespace std;
ifstream fin("aria.in");
ofstream fout("aria.out");
Punct Poligon[100003];
double Arie_triunghi(Punct A,Punct B,Punct C)
{
    return ((B.x-A.x)*(C.y-A.y)-(C.x-A.x)*(B.y-A.y))/2.0;
}
int main()
{
    int n,i;
    double arie=0;
    fin>>n;
    for(i=1;i<=n;i++) fin>>Poligon[i].x>>Poligon[i].y;
    Poligon[n+1]=Poligon[1];
    for(i=2;i<n;i++)
    {
        arie+=Arie_triunghi(Poligon[1],Poligon[i],Poligon[i+1]);
    }
    fout<<arie;
}