#include <fstream>
#include <cmath>
#include <iomanip>
using namespace std;
ifstream f("aria.in");
ofstream g("aria.out");
int i,n;
 double x,y,x2,y2,x3,y3;
 long double s;
int main()
{
    f>>n;
    f>>x>>y;
        f>>x2>>y2;
    for(i=3;i<=n;i++){
        f>>x3>>y3;
        s=s+abs(x*y2+y*x3+x2*y3-x2*y-x3*y2-x*y3);
        x2=x3;
        y2=y3;
    }
    g<<fixed<<setprecision(5)<<s/2;
    return 0;
}