#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> 
#include <ext/pb_ds/tree_policy.hpp> 
using namespace std;
using namespace __gnu_pbds;
typedef tree<int, null_type, less<int>, rb_tree_tag, tree_order_statistics_node_update> pbds;
#define ll long long int
#define ull unsigned long long int
#define ld long double
#define endl '\n'
#define loop(a,b,c) for(ll i=a;i<=b;i+=c)
#define intarr(arr,n) ll arr[n];for(ll i=0;i<n;i++)cin>>arr[i]
#define inparr(arr,n) for(ll i=0;i<n;i++)cin>>arr[i]
#define inpvec(vec,n) for(ll i=0;i<n;i++){ll var;cin>>var;vec.push_back(var);}
#define pb push_back
#define fastio ios_base::sync_with_stdio(false);cin.tie(NULL);
#define mod 1000000007
#define newline cout<<endl
#define ump unordered_map<ll,ll>
#define vec vector<ll>
#define mkp make_pair
#define disp(var1,var2) cout<<var1<<" "<<var2<<endl;
#define all(v)             v.begin(),v.end()
#define cout(var) cout<<var<<endl
#define mod2 998244353
#define fbo find_by_order  /// find kth smallest by passing k-1
#define ook order_of_key  //// find no of smaller items less than k
#define teensort(a,r,g,b) a[0]=r;a[1]=g;a[2]=b;sort(a,a+3);
#define ss second
#define ff first
#define displayarray(a,n) for(ll i=0;i<n;i++)cout<<a[i]<<" "; cout<<endl;
#define pi pair<ll,ll>
#define meramax 1e18
#define meramin -1e18
#define percentile 1000000007
#define PI 3.14159265358979323846

vector<string> v;
void bfs(auto g)
{
    string str="";
    string ans="";
    auto *visited = new bool[28]; 
    for(ll i=0;i<28;i++) 
        visited[i] = false; 
    list<ll> queue; 
    visited[27] = true; 
    queue.push_back(27); 
    str+="27";
    list<ll>::iterator i; 
    while(!queue.empty()) 
    { 
        auto s = queue.front();
        if(s==27)
        {
            str+="27";
        }
        else
        {
            char ch='A'+s; 
            str+=ch;
        }
        queue.pop_front(); 
        for (i = g[s].begin(); i != g[s].end(); ++i) 
        { 
            if (!visited[*i]) 
            { 
                visited[*i] = true; 
                queue.push_back(*i); 
            } 
        } 
    } 
    for(ll i=0;i<str.length();)
    {
        if(i+1<str.length() && str[i]=='2' && str[i+1]=='7')
        {
            i+=2;
            continue;
        }
        else
        {
            ans+=str[i];
            i++;
        }
    }
    v.pb(ans);
    //cout(ans);
    return;
}
void AcDegaYe()
{
    ll r,c;
    cin>>r>>c;
    char a[r+1][c+1];
    unordered_map<char,ll> h;
    for(ll i=1;i<=r;i++)
    {
        for(ll j=1;j<=c;j++)
        {
            cin>>a[i][j];
            h[a[i][j]]++;
        }
    }
    ll neeche[26];
    for(ll i=0;i<26;i++)neeche[i]=27;
    for(ll i=1;i<=r-1;i++)
    {
        for(ll j=1;j<=c;j++)
        {
            if(a[i][j]!=a[i+1][j])
            {
                neeche[a[i][j]-'A']=a[i+1][j]-'A';
            }
        }
    }
    unordered_map<ll,list<ll>> g;
    ll f=0;
    for(ll i=0;i<26;i++)
    {
        char ch='A'+i;
        if(h.count(ch)==1)
        {
            if(neeche[neeche[i]]==i)
            {
                f=1;
                break;
            }
            g[i].pb(neeche[i]);
            g[neeche[i]].pb(i);
        }
    }
    if(f)
    {
        v.pb("-1");
    }
    else
    {
        bfs(g);
    }
}
int main()
{   
    
    fastio
    ll t;
    cin>>t;
    //ll t=1;
    while(t--)
    {
        AcDegaYe();
    }
    for(ll i=1;i<=v.size();i++)
    {
        cout<<"Case #"<<i<<": "<<v[i-1]<<endl;
    }
    cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
    return 0;
}