#include<bits/stdc++.h>
#define rep(i,a,b) for(int i=int(a),_=int(b);i<=_;++i)
#define per(i,a,b) for(int i=int(a),_=int(b);i>=_;--i)
#define fi first
#define se second
using namespace std;

const int N=205,M=5005;
int T,n,m,u[M],v[M],mat[N],a[N][N],len,ans[N*M];
bool b[N][N],vis[N];

void Imp(int id){
	printf("Case #%d: IMPOSSIBLE\n",id);
}

bool aug(int x){
	if(vis[x]){
		return 0;
	}
	vis[x]=1;
	rep(i,1,n)if(b[x][i]&&(!mat[i]||aug(mat[i]))){
		return mat[i]=x,1;
	}
	return 0;
}

bool dfs(int v,int u){
	if(vis[v]){
		return 0;
	}
	vis[v]=1;
	if(mat[v]==u){
		return 1;
	}
	int w=mat[v];
	rep(i,1,n)if(b[w][i]&&dfs(i,u)){
		mat[i]=w;
		mat[v]=u;
		return 1;
	}
	return 0;
}

void euler(int u){
	rep(i,1,n)if(a[u][i]){
		--a[u][i];
		euler(i);
		ans[++len]=u;
	}
}

int main(){
	freopen("1.in","r",stdin);
	scanf("%d",&T);
	rep(id,1,T){
		scanf("%d%d",&n,&m);
		memset(a,0,sizeof(a));
		memset(b,0,sizeof(b));
		rep(i,1,m){
			scanf("%d%d",&u[i],&v[i]);
			b[u[i]][v[i]]=1;
		}
		memset(mat,0,sizeof(mat));
		int cnt=0;
		rep(i,1,n){
			memset(vis,0,sizeof(vis));
			cnt+=aug(i);
		}
		if(cnt<n){
			Imp(id);
			goto new_test;
		}
		rep(i,1,m){
			memset(vis,0,sizeof(vis));
			if(!dfs(v[i],u[i])){
				Imp(id);
				goto new_test;
			}
			rep(j,1,n){
				++a[mat[j]][j];
			}
		}
		len=0;
		ans[++len]=1;
		euler(1);
		printf("Case #%d: %d\n",id,len);
		per(i,len,1){
			printf("%d%c",ans[i]," \n"[i==1]);
		}
		new_test:;
	}
	return 0;
}