#include<stdio.h>

#include<string.h>

#include<iostream>

#include<algorithm>

#include<math.h>

#define N 110

#define INF 0xfffffff

using namespace std;

int f[N],vis[N];

int Find(int x)

{

    if(x!=f[x])

        f[x]=Find(f[x]);

    return f[x];

}

int main()

{

    int a,b,t=,pa,pb,flag,i,first;

    while(scanf("%d%d",&a,&b),a!=-||b!=-)

    {

        for(i=;i<N;i++)

            f[i]=i;

        memset(vis,,sizeof(vis));

        flag=;

        if(a==&&b==)

        {

            printf("Case %d is a tree.\n",t++);//空树也是树；

            continue;

        }

        vis[a] = vis[b] = ;//说明此点出现过；

        first = a;

        if(a == b) flag = ;

        pa = Find(a);

        pb = Find(b);

        if(pa != pb)

            f[pb] = pa;

        else

            flag = ;

        while(scanf("%d%d", &a, &b), a+b)

        {

            vis[a] = vis[b] = ;

            if(a == b) flag = ;

            pa = Find(a);

            pb = Find(b);

            if(pa != pb)

                f[pb]=pa;

            else

            {

                flag=;

            }

        }

        for(i=; i<N; i++)

        {

            if(vis[i]== && Find(i) != Find(first))

            {

                flag=;break;

            }

        }

        if(flag==)

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

        else

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

    }

    return ;

}