#include <bits/stdc++.h>
#define For(i,a,b) for(int i=a;i<=b;i++)
#define Rev(i,a,b) for(int i=a;i>=b;i--)
#define Fin(file) freopen(file,"r",stdin)
#define Fout(file) freopen(file,"w",stdout)
using namespace std; using ll = long long;
const int N=5e3+5,M=5e3+5;
int n,m,a[M],b[M],vis[N],cp[N],cp0[M][N],head[N],nxt[M*N],To[M*N],tot,lis[M*N],lcnt,Vis[N]; vector<int> to[N];
bool dfs(int u,int* CP){
	for(int v:to[u]) if(!vis[v]){
		vis[v]=1; if(!CP[v]||dfs(CP[v],CP)) return CP[v]=u,true;
	}
	return false;
}
void dfs0(int u){
	//~ cerr<<"u="<<u<<'\n';
	Vis[u]=1;
	for(int e=head[u];e;e=head[u]){
		head[u]=nxt[e]; dfs0(To[e]);
	}
	lis[++lcnt]=u;
}
int KASE;
void solve(){
	cin>>n>>m; For(i,1,n) to[i].clear(),cp[i]=Vis[i]=0;; For(i,1,m) { int x,y; cin>>x>>y; to[x].push_back(y); a[i]=x,b[i]=y; }
	cout<<"Case #"<<++KASE<<": ";
	int ok=1; For(i,1,n){
		memset(vis,0,sizeof(vis)); if(!dfs(i,cp)) { ok=0; break; }
	}
	if(!ok) { cout<<"IMPOSSIBLE\n"; return; }
	For(e,1,m){
		memcpy(cp0[e],cp,sizeof(cp0[e]));
		int x=a[e],y=b[e]; if(cp[y]==x) continue;
		int z=0; For(i,1,n) if(cp[i]==x) { z=i; break; }
		if(!cp[y]) cp0[e][z]=0,cp0[e][y]=x;
		else if(z==0) cp0[e][y]=x;
		else{
			int w=cp[y]; cp0[e][z]=0,cp0[e][y]=x;
			memset(vis,0,sizeof(vis)); vis[y]=1;
			if(!dfs(w,cp0[e])) return cout<<"IMPOSSIBLE\n",void();
		}
	}
	For(e,1,m){
		For(i,1,n) For(j,i+1,n) assert(cp0[e][i]!=cp0[e][j]);
		assert(cp0[e][b[e]]==a[e]);
	}
	//~ cout<<"OK\n";
	tot=0; For(i,1,n) head[i]=0;
	For(e,1,m) For(v,1,n){
		assert(cp0[e][v]);
		int u=cp0[e][v]; nxt[++tot]=head[u]; head[u]=tot; To[tot]=v; //printf("(%d,%d)\n",u,v);
	}
	lcnt=0; dfs0(1);
	For(i,1,n) if(!Vis[i]) return cout<<"IMPOSSIBLE\n",void();
	//~ For(i,1,lcnt) cout<<lis[i]<<' ';; out<<'\n';
	cout<<lcnt<<'\n'; Rev(i,lcnt,1) cout<<lis[i]<<' ';; cout<<'\n';
}
int main(){
	int T; cin>>T; while(T--) solve();
	return 0;
}