#include <cstdio>
        using namespace std;
        double arie (double x1, double y1, double x2, double y2)
        {
            double s=(x1*y2-x2*y1)/2;
            return s;
        }
        int main()
        {
            double n, i, X, Y, x, y, s, p, r, sum=0;
            freopen("aria.in","r",stdin);
            freopen("aria.out","w",stdout);
            scanf("%lf",&n);
            scanf("%lf%lf%lf%lf",&X,&Y,&x,&y);
            sum=arie(X,Y,x,y); p=x; r=y;
            for (i=1; i<=n-2; i++)
            {
                scanf("%lf%lf",&x,&y);
                s=arie(p,r,x,y);
                sum+=s  ; p=x; r=y;
            }
            sum+=arie(p,r,X,Y);
            printf("%.5lf\n",sum);
            return 0;
        }