#include <iostream>#include <cstdio>#include <cstdlib>#include <cstring>using namespace std;#define maxn 1006struct Edge{ int v, next;}edge[1000005];int name[maxn];int n, ecount, tot, root;int in[maxn];int head[maxn];bool vis[maxn];bool ok;void addedge(int a, int b){ edge[ecount].next = head[a]; edge[ecount].v = b; head[a] = ecount; in[b]++; ecount++;}int find(int a){ for (int i = 0; i < n; i++) if (name[i] == a) return i; name[n++] = a; return n - 1;}void dfs(int root){ vis[root] = true; tot++; for (int i = head[root]; i != -1; i = edge[i].next) { if (vis[edge[i].v]) { ok = false; return; } if (!ok) return; dfs(edge[i].v); }}int main(){ int t = 0; while (1) { t++; memset(in, 0, sizeof(in)); memset(head, -1, sizeof(head)); memset(vis, 0, sizeof(vis)); ecount = 0; n = 0; tot = 0; ok = true; int a, b; scanf("%d%d", &a, &b); if (a < 0 && b < 0) return 0; if (a == 0 && b == 0) { printf("Case %d is a tree.n", t); continue; } a = find(a); b = find(b); addedge(a, b); while (1) { scanf("%d%d", &a, &b); if (a == 0 && b == 0) break; a = find(a); b = find(b); addedge(a, b); } root = -1; for (int i = 0; i < n; i++) if (in[i] == 0) root = i; if (root >= 0) dfs(root); if (ok && tot == n && root >= 0) printf("Case %d is a tree.n", t); else printf("Case %d is not a tree.n", t); } return 0;}
