#include<stdio.h>
int main()
{
	int a[10000],b[10000],c[10000],m,n,i,j,L=0,k,s,p1,p2;
	for(;;)
	{
		s=0;
		k=0;
		L++;
		while(scanf("%d%d",&m,&n))
		{
			if(m==0&&n==0)
			{
				a[k]=m;
				b[k]=n;
				break;
			}
			if(m==-1&&n==-1)
				return 0;
			a[k]=m;
			b[k]=n;
			k++;
		}
		if(a[0]==0&&b[0]==0)
		{
			printf("Case %d is a tree.\n",L);
		}
		else
		{
			for(i=0;i<k;i++)
			{
				for(j=i+1;j<k;j++)
				{
					if(b[i]==b[j])
						break;
					if(b[i]==a[j]&&b[j]==a[i])
						break;
				}
				if(j!=k)
				{
					printf("Case %d is not a tree.\n",L);
					break;
				}
				if(a[i]==b[i])
				{
					printf("Case %d is not a tree.\n",L);
					break;
				}
				if(a[i]==a[i+1]&&b[i]==b[i+1])
				{
					printf("Case %d is not a tree.\n",L);
					break;
				}
				for(p1=0;p1<k;p1++)
				{
					for(p2=p1+1;p2<k;p2++)
						if(a[p1]==b[p2])
							break;
					if(p2==k)
					{
						c[s]=a[p1];
						s++;
					}
				}
				if(s==0)
				{
					printf("Case %d is not a tree.\n",L);
					break;
				}
				else
				{
					for(p1=0;p1<s-1;p1++)
					{
						if(c[p1]!=c[p1+1])
						{
							printf("Case %d is not a tree.\n",L);
							break;
						}
					}
					if(p1!=s-1)
						break;
				}
			}
			if(i==k)
				printf("Case %d is a tree.\n",L);
		}
	}
}