# include <stdio.h>
# include <math.h>

int main()
{
	int i, n;
	double V, S, a, b, c, p;
	double v1x, v1y, v2x, v2y, v3x, v3y;

	while(scanf("%d",&n)==1&&n>2)
	{
		S = 0;
		scanf("%lf%lf%lf%lf",&v1x,&v1y,&v2x,&v2y);
		for(i = 2; i < n; i++)
		{
			scanf("%lf%lf",&v3x,&v3y);
			a = sqrt((v1x-v2x)*(v1x-v2x)+(v1y-v2y)*(v1y-v2y));
			b = sqrt((v1x-v3x)*(v1x-v3x)+(v1y-v3y)*(v1y-v3y));
			c = sqrt((v3x-v2x)*(v3x-v2x)+(v3y-v2y)*(v3y-v2y));
			p = (a+b+c)/2.0;
			S += sqrt(p*(p-a)*(p-b)*(p-c));
			v2x = v3x; v2y = v3y;
		}
		scanf("%lf",&V);
		printf("BAR LENGTH: %.2lf\n",V/S);
	}
	return 1;
}