#include<bits/stdc++.h>
#define N 200
#define M 5000
using namespace std;
int T,n,m,match[N+5],u[M+5],v[M+5],st[N*M+5],top,head[N+5];
vector<int> e[N+5],E[N+5];
bool vis[N+5];
inline bool dfs(int x)
{
	for(int y:e[x]) if(!vis[y])
	{
		vis[y]=1;
		if(!match[y] || dfs(match[y]))
			return match[y]=x,1;
	}
}
inline void Dfs(int x)
{
	for(int &i=head[x];i<E[x].size();)
		Dfs(E[x][i++]),st[++top]=x;
}
inline void solve()
{
	for(int i=1;i<=n;++i)
	{
		memset(vis+1,0,n);
		if(!dfs(i)) return puts("IMPOSSIBLE"),void();
	}
	for(int i=1;i<=m;++i)
	{
		if(match[v[i]]!=u[i])
		{
			for(int j=1;j<=n;++j) if(match[j]==u[i]) {match[j]=0;break;}
			int temp=match[v[i]];match[v[i]]=u[i];
			memset(vis+1,0,n),vis[v[i]]=1;
			if(!dfs(temp)) return puts("IMPOSSIBLE"),void();
		}
		for(int j=1;j<=n;++j)
			E[match[j]].push_back(j);
	}
	st[top=0]=1,memset(head+1,0,n<<2),Dfs(1);
	if(top!=n*m) return puts("IMPOSSIBLE"),void();
	printf("%d\n",top+1);
	for(int i=top;~i;--i) printf("%d%c",st[i]," \n"[i==0]);
}
int main()
{
	scanf("%d",&T);
	for(int test=1;test<=T;++test)
	{
		scanf("%d %d",&n,&m);
		for(int i=1;i<=n;++i) e[i].clear(),E[i].clear();
		memset(match+1,0,n<<2);
		for(int i=1;i<=m;++i)
		{
			scanf("%d %d",u+i,v+i);
			e[u[i]].push_back(v[i]);
		}
		printf("Case #%d: ",test),solve();
	}
	return 0;
}