#include <iostream>
#include <memory.h>
using namespace std;

int father[5000];
bool has_point[5000];
int case_number;

void Ini_Set(int y){
     father[y]=y;
     has_point[y]=true;
}

int Find_Set(int x){
if(father[x]!=x){  //不是单独的节点
       father[x]=Find_Set(father[x]);
    }
return father[x];
}
void Union(int x,int y){   //s point to y
    x=Find_Set(x);
    y=Find_Set(y);
    father[y]=x;
}

int MaxOf(const int x,const int y){
return x>y?x:y;
}

int Begin(){

int s,t,max=0;
bool finish=false;
while(cin>>s>>t){
if(s>0 && t>0){
              Ini_Set(s);
              Ini_Set(t);
             max=MaxOf(MaxOf(max,s),t);  //找出节点值最大的点,注意用法
             if(Find_Set(s)!=Find_Set(t))  
                 Union(s,t);
else{       //存在重复路径
                  cout<<"Case "<<case_number<<" is not a tree."<<endl;
                  finish=true;
break;
             }
          }
else{
if(s==-1 && t==-1){
return 1;
              }
break;
          }
      }
if(!finish){
int f=Find_Set(max);
for(int i=1;i<=max;++i)
if(has_point[i] && Find_Set(i)!=f){  //是森林
                 cout<<"Case "<<case_number<<" is not a tree."<<endl;
                 finish=true;
break;
             }
      }
if(!finish)
          cout<<"Case "<<case_number<<" is a tree."<<endl;
return 0;
}

int main(){
while(true){
      ++case_number;
if(Begin())
return 0;
    }
return 0;
}