#include <iomanip>
#include <fstream>

using namespace std;

int n ;
double s ;
double x[1000007] , y[1000007] ;

int main()
{
    ifstream cin ("aria.in");
    ofstream cout ("aria.out");
    cin>>n;
    for ( int i = 1 ; i <= n ; ++i )
        cin>>x[i]>>y[i];
    x[n+1]=x[1];
    y[n+1]=y[1];
    for ( int i = 1 ; i <= n ; ++i )
        {
            s+=x[i]*y[i+1]-x[i+1]*y[i];
        }
    cout<<fixed<<setprecision(6)<<s/2.0<<"\n" ;
    return 0;
}