#include <fstream>

using namespace std;

struct punct
{
   double x;
   double y;
}puncte[100005];

double arie(double x1,double y1,double x2,double y2)
{
    return (((x1*y2)-(x2*y1))/2); 
}

int main()
{
    ifstream fin("aria.in");
    ofstream fout("aria.out");
    
    double a=0;
    int n,i;
    double x1,y1;
    fin>>n;
    
    fin>>x1>>y1;
    puncte[0].x=x1;
    puncte[0].y=y1;
    
    for(i=1;i<n;i++)
    {            
        fin>>x1>>y1;
        puncte[i].x=x1;
        puncte[i].y=y1;
        a+=(arie(puncte[i-1].x,puncte[i-1].y,puncte[i].x,puncte[i].y));         
    }
    
    a+=(arie(puncte[n-1].x,puncte[n-1].y,puncte[0].x,puncte[0].y));
    
    fout<<a<<'\n';
    
    fin.close();
    fout.close();
    //system("PAUSE");
    return 0;
}