#include<stdio.h>
#include<stdlib.h>
#define INF 20
int Tree[INF+1][INF+1];
int In[INF+1],Out[INF+1];
int IsTree()
{
    int i,flag,count;
    flag=0;
    for(i=1;i<=INF;i++)
    {
       if(In[i]>1) break;
       if(In[i]==0&&Out[i]!=0&&flag) break;
       if(In[i]==0&&Out[i]!=0) flag=1; //查根
    }
    if((i>INF)&&(flag==1)) return 1;
    return 0;
}
main()
{
    int a,b,i,j,count,flag,flag_kong;
    count=1;
    while(1)
    {
       flag=1;flag_kong=1;
       for(i=0;i<=INF;i++)
       {
             In[i]=Out[i]=0;
             for(j=0;j<=INF;j++) Tree[i][j]=0;
       }
       scanf("%d%d",&a,&b);
       if(a==-1&&b==-1) break;
       while(1)
       {
          if(a==0&&b==0) break;
          flag_kong=0;
          if(a==b||Tree[b][a]) flag=0;
          if(Tree[a][b]==0)
          {
                Tree[a][b]=1;
                In[b]++; Out[a]++;
          }
          scanf("%d%d",&a,&b);
       }
       if(flag_kong)
          printf("Case %d is a tree.\n",count++);
       else
       {
          if(flag==0)
             printf("Case %d is not a tree.\n",count++);
          else
             if(IsTree())
                 printf("Case %d is a tree.\n",count++);
             else
                 printf("Case %d is not a tree.\n",count++);
        }
    }
    system("pause\n");
    return 0;
}