#include <cstdio>

 #include <cstring>

 #include <iostream>

 using namespace std;

 const int N = ;

 int in[N] , first[N] , k , maxn , del , sum;//maxn记录总共的点的个数，del记录删除了的点的个数

 struct Edge{

     int y , next;

 }e[N<<];

 void add_edge(int x , int y )

 {

     e[k].y = y , e[k].next = first[x];

     first[x] = k++;

 }

 void tuopu(int src)

 {

     del++;

     for(int i = first[src] ; i!=- ; i=e[i].next){

         int v = e[i].y;

         in[v]--;

         if(!in[v]) tuopu(v);

     }

 }

 int main()

 {

    // freopen("a.in" , "r" , stdin);

     int x , y , cas = ;

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

         if(x <  && y < ) break;

         memset(first , - , sizeof(first));

         memset(in , - , sizeof(in));

         k =  , maxn =  , sum = ;//sum表示总共点的个数

         int  cnt =  , root , flag = ; //cnt记录有多少节点可以作为顶点了,flag作为判断是否有点超过1个入度

         while(x !=  || y!=){

             add_edge(x , y);

             maxn = max(max(maxn , x) , y);

             if(in[x] < ) in[x] =  , sum++;

             if(in[y] < ) in[y] =  , sum++;

             in[y]++;

             if(in[y] >= ){

                 flag = ;

             }

             scanf("%d%d" , &x , &y);

         }

        // cout<<"maxn: "<<maxn<<endl;

         if(flag){

           //  cout<<"有点超过两个入度 "<<endl;

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

             continue;

         }

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

             if(!in[i]){

                 root = i;

                 cnt++;

                 if(cnt>=) break;

             }

         if(cnt >= ){

            // cout<<"有超过两个点可作为树根 "<<endl;

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

             continue;

         }

         del = ;

         tuopu(root);

         if(del < sum)

         {

            // cout<<"出现内部环 "<<endl;

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

         }

         else

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

     }

     return ;

 }