#include<bits/stdc++.h>
using namespace std;
const int maxn=5e5+10;
vector<int>G[maxn],ed[maxn];
int p[maxn],vis[maxn],n,m,U[maxn],V[maxn];
bool hun(int x){
    for(auto v:ed[x]){
        if(!vis[v]){
            vis[v]=1;
            if(!p[v]||hun(p[v])){
                p[v]=x;
                return true;
            }
        }
    }
    return false;
}
int S;
bool adj(int x){
    vis[x]=1;
    if(p[x]==S)return true;
    for(auto v:ed[p[x]]){
        if(vis[v]||!adj(v))continue;
        p[v]=p[x],p[x]=S;
        return true;
    }
    return false;
}
vector<int>ans;
void dfs(int x){
    while(G[x].size()){
        int v=G[x].back();
        G[x].pop_back(),dfs(v);
        ans.push_back(x);
    }
}
void sol(){
    cin>>n>>m;
    for(int i=1;i<=n;i++)ed[i].clear(),G[i].clear(),p[i]=0;
    for(int i=1;i<=m;i++){
        cin>>U[i]>>V[i];
        ed[U[i]].push_back(V[i]);
    }
    int cnt=0;
    for(int i=1;i<=n;i++){
        for(int j=1;j<=n;j++)vis[j]=0;
        if(hun(i))cnt++;
    }
    if(cnt<n){
        cout<<"IMPOSSIBLE"<<"\n";
        return;
    }
    for(int i=1;i<=m;i++){
        for(int j=1;j<=n;j++)vis[j]=0;
        S=U[i];
        if(!adj(V[i])){
            cout<<"IMPOSSIBLE"<<"\n";
            return;
        }
        for(int j=1;j<=n;j++)G[p[j]].push_back(j);
    }
    ans.push_back(1),dfs(1),reverse(ans.begin(),ans.end());
    if(ans.size()!=n*m+1){
        cout<<"IMPOSSIBLE"<<"\n";
        return;
    }
    cout<<ans.size()<<"\n";
    for(auto v:ans)cout<<v<<" ";
    cout<<"\n";
}
int main(){
    cin.tie(0),cout.tie(0),ios::sync_with_stdio(false);
    int tc;
    cin>>tc;
    for(int i=1;i<=tc;i++){
        cout<<"Case #"<<i<<": ";
        sol();
    }
    return 0;
}