/*This Code is Submitted by billforum for Problem 2183 at 2012-02-20 15:36:33*/
#include <iostream>
#include <memory.h>
#include <stdlib.h>
#include <cstdio>
#include <cstring>
#include <cmath>
using namespace std;
int main()
{
int n;
double x0,y0,x1,y1,x2,y2;
double ans,v;
while(scanf("%d",&n)!=EOF)
{
if(n<3) break;
ans=0;
scanf("%lf%lf%lf%lf",&x0,&y0,&x1,&y1);
for(int i=1;i<=n-2;i++)
{
scanf("%lf%lf",&x2,&y2);
ans+=(x0*y1+x1*y2+x2*y0-y0*x1-y1*x2-y2*x0);
x1=x2;
y1=y2;
}
if(ans<0) ans=-ans;
scanf("%lf",&v);
printf("BAR LENGTH: %.2lf\n",2*v/ans);
}
return 0;
}