/*
6 8 5 3 5 2 6 4
5 6 0 0
8 1 7 3 6 2 8 9 7 5
7 4 7 8 7 6 0 0
3 8 6 8 6 4
5 3 5 6 5 2 0 0

1 2 3 2 4 2 5 2 0 0
1 2 3 4  4 3 0 0
0 0 
1 1 0 0
1 2 2 1 0 0
1 2 2 3 3 4 4 1 0 0
1 2 2 3 3 1 5 6 0 0
2 3 0 0
-1 -1
*/

/*
Case 1 is a tree.
Case 2 is a tree.
Case 3 is not a tree.
Case 4 is not a tree.
Case 5 is not a tree.
Case 6 is a tree.
Case 7 is not a tree.
Case 8 is not a tree.
Case 9 is not a tree.
Case 10 is not a tree.
Case 11 is a tree.

*/

#include<stdio.h>
#include<string.h>
#include<stdlib.h>
bool hasp[10000];
struct node{
  int father,rank;
};
node root[10000];
void init(){
  int i;
  root[0].rank=0;
  root[0].father=0;
  for(i=1;i<10000;i++){
      root[i].father=i;
      root[i].rank=1;
  }
}
int find(int a){
    while(a!=root[a].father)
         a=root[a].father;
    return a;
}
void Union(int a,int b){
    /*a->b*/
    root[b].father=a;
    root[a].rank+=root[b].rank;
}
int main(){
    freopen("test.in","r",stdin);
    //system("call test.in");
    int a,b,cnt,x,y,cas=1,tem;
    bool flag;
    while(1){
          flag=false;   //初始化为无环
      memset(hasp,0,sizeof(hasp));
      tem=cnt=0;
      init();
      while(scanf("%d%d",&a,&b)&&(a+b!=0)){
        if(a+b<0) return 0;
        if(!flag)
        {
          x=find(a);
          y=find(b);
         if(x==y) flag=1;
         else Union(a,b);
        if(tem==0) tem=a;
       if(!hasp[a]) hasp[a]=1 , cnt++ ;
       if(!hasp[b]) hasp[b]=1 , cnt++ ;
        }
    }
     /* cnt记录了点的个数 */
     if(root[find(tem)].rank==cnt&&!flag)
        printf("Case %d is a tree.\n",cas++);
     else
        printf("Case %d is not a tree.\n",cas++);
    }
  return 0;
}