#include<stdio.h>
#include<string.h>
#define MAX 10000
bool  graph[100][100];
bool  hash[100], isleaf[100];
int  root, maxid;
int  edges, nodes;
int  init(int  a,  int  b)
{
    int  i;
    nodes = 0; edges = 0;
    root = -1;  maxid = 0;
    memset(graph, 0, sizeof(graph));
    memset(hash, 0, sizeof(hash));
    while(a+b != 0)
    {
        graph[a][b] = 1;
        if(!hash[a]) nodes++;
        if(!hash[b]) nodes++;
        edges++;
        hash[a] = 1;
        hash[b] = 1;
        isleaf[b] = 1;
        if(a > maxid) maxid = a;
        if(b > maxid) maxid = b;
        scanf("%d %d", &a, &b);
    }
    for(i = 0; i <= maxid; i++)
        if(!isleaf[i] && hash[i])
        {
            if(root != -1) return  -1;
            root  = i;
        }
    if(edges+1 != nodes) root = -1;
    return  root;
}
int  BFS(int  root)
{
    int  queue[MAX];
    int  rear = 1, front = 0, i;
    int  cnt = 1;
    memset(hash, 0, sizeof(hash));
    queue[0] = root;
    while(rear != front)
    {
        int  cur = queue[front];
        front = (front+1)%MAX;
        for(i = 0; i <= maxid; i++)
        {
            if(!hash[i] && graph[cur][i])
            {
                queue[rear] = i;
                hash[i] = 1;
                cnt ++;
                rear = (rear+1)%MAX;
            }
        }
    }
    if(cnt == nodes) return  1;
    else  return  0;
}

int  main()
{
    int  a, b, cs = 1;
    while(scanf("%d %d", &a, &b) && (a != -1 && b !=-1))
    {
        //if(a == b && a == 0) {printf("Case %d is a tree.\n", cs++); continue;}
        root = init(a, b);
        //printf("root = %d\n", root);
        if(root != -1 && BFS(root))
            printf("Case %d is a tree.\n", cs++);
        else  printf("Case %d is not a tree.\n", cs++);
    }
    return  0;
}