#include <cstdio>
#include <cmath>
using namespace std;
int n;
double x1,x2,y1,y2,ans;
int main()
{
    freopen("aria.in","r",stdin);
    freopen("aria.out","w",stdout);
    int i;
    double xx,yy;
    scanf("%d",&n);
    scanf("%lf%lf",&x1,&y1);
    xx=x1;
    yy=y1;
    for(i=2;i<=n;i++)
        {
            scanf("%lf%lf",&x2,&y2);
            ans+=0.5*(x1*y2-x2*y1);
            x1=x2;
            y1=y2;
        }
    ans+=0.5*(x1*yy-xx*y1);
    printf("%.5f\n",fabs(ans));
    return 0;
}