#include<cstdio>
#include<iostream>
#include<queue>
using namespace std;
const int o=1e6+10;
int Tc,n,m,S,T,h[o],cur[o],cnt,d[o],ind[o],outd[o],flow,u[o],v[o],id[o],st[o],tp;queue<int> q;
struct Edge{int v,p,c;}e[o];
inline void ad_(int U,int V){e[++cnt].v=V;e[cnt].p=h[U];h[U]=cnt;}
inline void ad(int U,int V,int C){e[++cnt].v=V;e[cnt].p=h[U];e[h[U]=cnt].c=C;}
inline void add(int ST,int ED,int CA){ad(ST,ED,CA);ad(ED,ST,0);}
inline bool bfs(){
	for(int i=S;i<=T;++i) d[i]=0,cur[i]=h[i];
	for(q.push(S),d[S]=1;!q.empty();q.pop()) for(int i=h[q.front()];i;i=e[i].p)
		if(e[i].c&&!d[e[i].v]) d[e[i].v]=d[q.front()]+1,q.push(e[i].v);
	return d[T];
}
int dfs(int A,int B){
	if(A==T||!B) return B;
	int res=0,x;
	for(int&i=cur[A];i;i=e[i].p) if(e[i].c&&d[e[i].v]==d[A]+1)
		{x=dfs(e[i].v,min(B-res,e[i].c));e[i].c-=x;e[i^1].c+=x;if((res+=x)==B) return B;}
	return res;
}
void Dfs(int nw){int t;for(int&i=h[nw];i;) t=e[i].v,i=e[i].p,Dfs(t),st[++tp]=nw;}
inline void slv(){
	scanf("%d%d",&n,&m);S=0;T=n*2+1;cnt=1;flow=0;
	for(int i=S;i<=T;++i) h[i]=0;
	for(int i=1;i<=n;++i) ind[i]=outd[i]=0;
	for(int i=1;i<=m;++i) scanf("%d%d",&u[i],&v[i]),++ind[v[i]],++outd[u[i]],add(u[i],v[i]+n,o),id[i]=cnt;
	for(int i=1;i<=n;++i){
		if(ind[i]>m||outd[i]>m){printf("IMPOSSIBLE\n");return;}
		add(i+n,T,m-ind[i]);add(S,i,m-outd[i]);flow+=m-ind[i];
	}
	for(;bfs();flow-=dfs(S,o));
	if(flow){printf("IMPOSSIBLE\n");return;}
	cnt=0;
	for(int i=1;i<=n;++i) h[i]=0;
	for(int i=1;i<=m;++i) for(int j=e[id[i]].c+1;j--;) ad_(u[i],v[i]);
	st[tp=0]=1;Dfs(1);
	printf("%d\n",tp+1);
	for(int i=tp;i+1;--i) printf("%d ",st[i]);
	putchar('\n');
}
int main(){
	scanf("%d",&Tc);
	for(int i=1;i<=Tc;++i) printf("Case #%d: ",i),slv();
	return 0;
}