#include<iostream>

 #include<stack>

 #include<queue>

 #include<iomanip>

 #include<stdio.h>

 #include<algorithm>

 #include<string>

 #include<cstring>

 #include<cmath>

 #include<map>

 using namespace std;

 # define inf 0x3f3f3f3f

 # define maxn

 int father[maxn];

 int vis[maxn];

 int k;

 map<int,int>q;

 int Find(int t)

 {

     return t==father[t]?t:father[t]=Find(father[t]);

 }

 void match(int t1,int t2)

 {

     int x=Find(t1);

     int y=Find(t2);

     if(x!=y)

     {

         father[y]=x;

         return ;

     }

     else

         k=;

 }

 int main()

 {

     int x,y;

     int num=;

     while(~scanf("%d %d",&x,&y))

     {

         q.clear();

         k=;

         if(x==-&&y==-)break;

         memset(vis,,sizeof(vis));

         if(x==&&y==)

         {

             printf("Case %d is a tree.\n",++num);

             continue;

         }

         for(int i=; i<=maxn; i++)

         {

             father[i]=i;

         }

         vis[x]=;

         vis[y]=;

         match(x,y);

         while(~scanf("%d%d",&x,&y))

         {

             if(x==&&y==)break;

             match(x,y);

             vis[x]=;

             vis[y]=;

         }

         if(k==)printf("Case %d is not a tree.\n",++num);

         else

         {

             int flag=;

             for(int i=; i<=maxn; i++)

             {

                 if(vis[i])

                 {

                     int temp=Find(i);

                     q[temp]=;

                 }

             }

             if(q.size()==) printf("Case %d is a tree.\n",++num);

                 else printf("Case %d is not a tree.\n",++num);

         }

     }

     return ;

 }