#include
     
      #include
      
       #include
       
        using namespace std;#define N 5001struct Point{int x,y;}p[N],bao[N];bool operator < (Point a,Point b){return a.x!=b.x?a.x
        
         
          1&&Cross(bao[en]-bao[en-1],p[i]-bao[en])<=0)	  	  --en;	  	bao[++en]=p[i];	  }	int half=en;	for(int i=n-1;i;--i)	  {	  	while(en>half&&Cross(bao[en]-bao[en-1],p[i]-bao[en])<=0)	  	  --en;	  	bao[++en]=p[i];	  }	for(int i=2;i<=en;++i)	  ans+=dis(bao[i],bao[i-1]);	printf("%.2f\n",ans+dis(bao[1],bao[en]));	return 0;}