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