UVA 615

#define ll long long
#include <bits/stdc++.h>
using namespace std;

const int OO = 1e9;
const double EPS = 1e-9;

map<int,bool> vis;
set<int> nodes;
map<int,vector<int>> graph;
map<int,int> indeg;

bool dfs(int x) {
    //cout << "here at x = " << x << "\n";
    if(vis[x]) {
        return false;
    }
    vis[x] = 1;
    bool ret = true;
    for(int e : graph[x]) {
        ret = dfs(e) && ret;
    }
    return ret;
}

int main()
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout.tie(NULL);
    int a,b;
    int ti = 0;
    while(true) {
        cin >> a >> b;
        if(a < 0 && b < 0) {
            break;
        }
        ti++;
        vis.clear();
        nodes.clear();
        graph.clear();
        indeg.clear();
        while(true) {
            if(a == 0 && b == 0) {
                break;
            }
            graph[a].push_back(b);
            indeg[b]++;
            nodes.insert(a);
            nodes.insert(b);
            cin >> a >> b;
        }
        int root = -1;
        bool tree = true;
        if(nodes.empty()) {
            goto done;
        }

        for(int x : nodes) {
            if(indeg[x] == 0) {
                if(root != -1) {
                    tree = false;
                    goto done;
                }
                root = x;
            }
        }
        if(root == -1) {
            tree = false;
            goto done;
        }

        tree = tree && dfs(root);
        for(int x : nodes) {
            if(!vis[x]) {
                tree = false;
                break;
            }
        }
        done:
            cout << "Case " << ti << " is " << (tree ? "a tree.\n":"not a tree.\n");
    }
    return 0;
}