#include <bits/stdc++.h>
using namespace std;
const int N=510,M=1e5; 
class Maxflow{
private:
	int nedge=1,p[2*M],nex[2*M],head[N],c[2*M],cur[2*M];
    int dist[2*N];
    int S,T;
	void Addedge(int a,int b,int v){
		p[++nedge]=b;nex[nedge]=head[a];head[a]=nedge;
		c[nedge]=v;
	}
    bool bfs(){
		queue<int>q;
		for(int i=S;i<=T;i++)dist[i]=-1;
		dist[S]=0;q.push(S);
		while(!q.empty()){
			int now=q.front();q.pop();
			for(int k=head[now];k;k=nex[k])if(dist[p[k]]==-1&&c[k]>0){
				dist[p[k]]=dist[now]+1;
				q.push(p[k]);
			}
		}
		return dist[T]>-1;
	}
	int dfs(int x,int low){
		if(x==T)return low;
		if(low==0)return 0;
		int used=0;
		for(int &k=cur[x];k;k=nex[k])if(dist[p[k]]==dist[x]+1&&c[k]>0){
			int a=dfs(p[k],min(c[k],low-used));
			c[k]-=a;c[k^1]+=a;used+=a;
			if(low==used)break;
		}
		if(used==0)dist[x]=-1;
		return used;
	}
public:
	void init(int s,int t){
		for(int i=S;i<=T;i++)head[i]=0;
		S=s,T=t;
		nedge=1;
	}
    void addedge(int a,int b,int v){
        Addedge(a,b,v);
        Addedge(b,a,0);
	}
	int dinic(vector<pair<int,int>> &match){
		int flow=0;
		while(bfs()){
			for(int i=S;i<=T;i++)cur[i]=head[i];
			flow+=dfs(S,1e9);
		}
        for(int i=1;i<=T/2;i++){
            for(int k=head[i];k;k=nex[k])if(p[k]>T/2&&p[k]<T&&c[k]==0){
                match.emplace_back(i,p[k]);
            }
        }
		return flow;
	}
}sol;
vector<int> getpath(int n,vector<pair<int,int>> &edges,bool &flag){
    vector<vector<int>> ve(n+1);
    for(auto &[x,y]:edges)ve[x].push_back(y);
    vector<int> ans;
    vector<int> vis(n+1);
    function<void(int)> dfs = [&](int u)//????
    {
        vis[u]=1;
        while(!ve[u].empty())
        {
            int v=ve[u].back();ve[u].pop_back();
            dfs(v);
        }
        ans.push_back(u);
    };
    dfs(1);
    flag=true;
    for(int i=1;i<=n;i++)flag&=vis[i];
    reverse(ans.begin(),ans.end());
    return ans;
}
const bool test=false;
void solve()
{
    int n,m;
    if(!test)cin>>n>>m;
    else n=10,m=20;
    int s=0,t=2*n+1;
    sol.init(s,t);
    vector<pair<int,int>> edges(m);
    vector<vector<int>> lis(n+1);
    for(int i=0;i<m;i++)cin>>edges[i].first>>edges[i].second,edges[i].second+=n,lis[edges[i].first].push_back(edges[i].second);
    for(int i=1;i<=n;i++)sol.addedge(s,i,1),sol.addedge(i+n,t,1);
    for(auto [x,y]:edges)sol.addedge(x,y,1);
    vector<pair<int,int>> match;
    if(sol.dinic(match)!=n){cout<<"IMPOSSIBLE\n";return;}
    vector<int> to(n+1);
    vector<pair<int,int>> all;
    for(auto [x,y]:match)to[x]=y;
    // cout<<"match :";for(auto [x,y]:match)cout<<x<<"/"<<y<<" ";;cout<<endl;
    vector<vector<int>> ve(2*n+1);
    for(auto [x,y]:edges){
        if(y==to[x])ve[x].push_back(y);
        else ve[y].push_back(x);
    }
    for(int rt=1;rt<=n;rt++){
        vector<int> fa(n*2+1,-1);
        queue<int> qu;qu.push(rt);
        while(!qu.empty()){
            int x=qu.front();qu.pop();
            for(auto it:ve[x])if(fa[it]==-1){
                fa[it]=x;
                qu.push(it);
            }
        }
        for(auto it:lis[rt]){
            if(fa[it]==-1){cout<<"IMPOSSIBLE\n";return;}
            auto tt=to;
            int x=it;
            vector<int> v;
            while(x!=rt){
                v.push_back(x);x=fa[x];
            }
            v.push_back(rt);
            reverse(v.begin(),v.end());
            for(int i=2;i<(int)v.size();i+=2)tt[v[i]]=v[i-1];
            tt[rt]=v.back();
            for(int i=1;i<=n;i++)all.emplace_back(i,tt[i]-n);
        }
    }
    bool flag=true;
    auto ans=getpath(n,all,flag);
    if(!flag){cout<<"IMPOSSIBLE\n";return;}
    for(int i=1;i<(int)ans.size();i++){
        int p=ans[i-1],q=ans[i];
        assert(find(lis[p].begin(),lis[p].end(),q+n)!=lis[p].end());
    }
    cout<<ans.size()<<"\n";
    for(auto it:ans)cout<<it<<" ";;cout<<endl;
}
int main(){
    ios::sync_with_stdio(false);
    cin.tie(0);
    int T;
    cin>>T;
    for(int ts=1;ts<=T;ts++){
        cout<<"Case #"<<ts<<": ";
        solve();
    }
    return 0;
}