package v006;

import java.io.BufferedReader;

import java.io.IOException;

import java.io.InputStream;

import java.io.InputStreamReader;

import java.io.PrintWriter;

import java.util.ArrayList;

import java.util.StringTokenizer;

import java.util.TreeMap;

public class IsItATree_UVa615 {

static boolean dfs(Node u)

{

u.visited = true;

for(Node v: u.children)

if(v.visited || !dfs(v))

return false;

return true;

}

public static void main(String[] args) throws IOException

{

Scanner sc = new Scanner(System.in);

PrintWriter out = new PrintWriter(System.out);

int tc = 1;

while(true)

{

int u = sc.nextInt() - 1, v = sc.nextInt() - 1;

if(u < -1)

break;

TreeMap<Integer, Node> map = new TreeMap<Integer, Node>();

while(u != -1 && v != -1)

{

Node x = map.get(u), y = map.get(v);

if(x == null)

map.put(u, x = new Node());

if(y == null)

map.put(v, y = new Node());

x.children.add(y);

y.isRoot = false;

u = sc.nextInt() - 1;

v = sc.nextInt() - 1;

}

Node root = null;

boolean isTree = true;

for(Node node: map.values())

if(node.isRoot)

{

root = node;

break;

}

if(map.size() > 1 && (root == null || !dfs(root)))

isTree = false;

for(Node node: map.values())

if(!node.visited)

isTree = false;

if(isTree)

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

else

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

}

out.flush();

out.close();

}

static class Node

{

boolean isRoot = true, visited;

ArrayList<Node> children = new ArrayList<Node>();

}

static class Scanner

{

StringTokenizer st;

BufferedReader br;

public Scanner(InputStream s){ br = new BufferedReader(new InputStreamReader(s));}

public String next() throws IOException

{

while (st == null || !st.hasMoreTokens())

st = new StringTokenizer(br.readLine());

return st.nextToken();

}

public int nextInt() throws IOException {return Integer.parseInt(next());}

public long nextLong() throws IOException {return Long.parseLong(next());}

public String nextLine() throws IOException {return br.readLine();}

public double nextDouble() throws IOException

{

String x = next();

StringBuilder sb = new StringBuilder("0");

double res = 0, f = 1;

boolean dec = false, neg = false;

int start = 0;

if(x.charAt(0) == '-')

{

neg = true;

start++;

}

for(int i = start; i < x.length(); i++)

if(x.charAt(i) == '.')

{

res = Long.parseLong(sb.toString());

sb = new StringBuilder("0");

dec = true;

}

else

{

sb.append(x.charAt(i));

if(dec)

f *= 10;

}

res += Long.parseLong(sb.toString()) / f;

return res * (neg?-1:1);

}

public boolean ready() throws IOException {return br.ready();}

}

}