#include <cstdio>

int n,i;
double x1,y1,x2,y2,xx1,yy1,aria;

int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    scanf("%d",&n);
    scanf("%lf%lf",&x1,&y1);
    xx1=x1;
    yy1=y1;
    for (i=2; i<=n; i++)
    {
        scanf("%lf%lf",&x2,&y2);
        aria=aria+x1*y2-x2*y1;
        x1=x2;
        y1=y2;
    }
    aria=aria+x2*yy1-xx1*y2;
    aria=aria/2;
    if (int(aria)==aria) printf("%.0lf\n",aria);
        else printf("%.6lf",aria);
    return 0;
}