#include<bits/stdc++.h>
#define pii pair<int,int>
#define fi first
#define se second
#define mp make_pair
#define pb push_back
using namespace std;
const int N=1e6+100;
int n,m;
int vis[N],lc[N],rc[N];
vector<int>g[N];
int dfs(int x)
{
	for(auto y:g[x])
	{
		if(vis[y])continue;vis[y]=1;
		if(rc[y]==-1||dfs(rc[y]))
		{
			lc[x]=y;rc[y]=x;
			return 1;
		}
	}
	return 0;
}
vector<pii>e;
stack<int>st;
vector<int>f[N];
void dfs1(int x)
{
	while(!f[x].empty())
	{
		int y=f[x].back();f[x].pop_back();
		dfs1(y);
	}
	st.push(x);
}
void solve()
{
	scanf("%d%d",&n,&m);e.clear();
	while(!st.empty())st.pop();
	for(int i=1;i<=n;i++)g[i].clear(),f[i].clear(),lc[i]=rc[i]=-1;
	for(int i=1,x,y;i<=m;i++)
	{
		scanf("%d%d",&x,&y);
		g[x].pb(y);e.pb(mp(x,y));
	}
	for(int i=1;i<=n;i++)
	{
		for(int j=1;j<=n;j++)vis[j]=0;
		if(!dfs(i))return puts("IMPOSSIBLE"),void();
	}
	for(auto t:e)
	{
		int x,y;tie(x,y)=t;
		if(lc[x]!=y)
		{
			rc[lc[x]]=-1;
			for(int i=1;i<=n;i++)vis[i]=0;vis[y]=1;
			if(!dfs(rc[y]))return puts("IMPOSSIBLE"),void();
			else lc[x]=y,rc[y]=x;
		}
		for(int i=1;i<=n;i++)f[i].push_back(lc[i]);
	}
	dfs1(1);
	if(st.size()!=n*m+1)return puts("IMPOSSIBLE"),void();
	printf("%d\n",(int)st.size());
	while(!st.empty())printf("%d ",st.top()),st.pop();
}
int main()
{
	int _;scanf("%d",&_);
	for(int i=1;i<=_;i++)
		printf("Case #%d: ",i),solve();
	return 0;
}
/*
1
4 6
1 2
1 4
2 3
3 2
3 4
4 1
*/