#include<bits/stdc++.h>
using namespace std;
int gcd(int a, int b) { if (a == 0) return b; return gcd(b % a, a);}
#define MOD 1000000007
#define MOD2 998244353
#define INF (1<<29)
#define LINF (1LL<<60)
#define EPS (1e-10)
#define PI 3.1415926535897932384626433832795028
#define ll long long int
bool codejam = 1;

void topologicalSortUtil(int v, bool visited[], stack<int> &Stack,vector<vector<int>> adj) 
{
    visited[v] = true;  
    vector<int>::iterator i; 
    for (i = adj[v].begin(); i != adj[v].end(); ++i) 
        if (!visited[*i]) 
            topologicalSortUtil(*i, visited, Stack,adj); 
    Stack.push(v); 
} 

void topologicalSort(ll n,vector<vector<int>> adj) 
{ 
	ll V=n;
    stack<int> Stack; 
    bool *visited = new bool[V]; 
    for (int i = 0; i < V; i++) 
        visited[i] = false; 
    for (int i = 0; i < V; i++) 
        if (visited[i] == false and adj[i].size()!=0) 
            topologicalSortUtil(i, visited, Stack,adj); 
    while (Stack.empty() == false) 
    { 
        cout << (char) ('A' + Stack.top()); 
        Stack.pop(); 
    } 
} 

void printOrder(vector<string>& words, int n, int alpha) 
{ 
	vector<vector<int>> adj(26,vector<int>{});
    for (int i = 0; i < words.size(); i++) 
    {	
    	for(int j=0;j<words[i].size()-1;j++)
    		adj[words[i][j]-'A'].push_back(words[i][j+1]-'A');
    }
    topologicalSort(n,adj); 
} 

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(0);
    cout.tie(0);
	#ifndef ONLINE_JUDGE
		freopen("input.txt","r",stdin);
		freopen("output.txt","w",stdout);
	#endif

	ll t;
	cin>>t;
	ll tc = 1;
	while(t--){
		if(codejam){
			cout<<"Case #"<<tc++<<": ";
		}
		ll r,c;
		cin>>r>>c;
		vector<vector<char>> a(r,vector<char>(c,'A'));
		map<int,map<ll,ll>> m;
		unordered_set<char> alpha;
		for(int i=0;i<r;i++){
			for(int j=0;j<c;j++){
				cin>>a[i][j];
				alpha.insert(a[i][j]);
				m[j][a[i][j]-'A']++;
			}
		}
		bool failed = false;
		for(int j=0;j<c;j++){
			for(int row = r-1;row>=0;row--){
				int count = 1;
				int count2 = 0;
				while(row>0 and a[row-1][j] == a[row][j]){
					count++;
					row--;
				}
				row = r-1;
				while(row>=0){
					if(a[row][j]==a[r-1][j])
						count2++;
					row--;
				}
				if(count != count2){
					cout<<-1<<endl;
					failed = true;
					break;
				}
			}
			if(failed)
				break;
		}
		if(!failed){
			vector<string> words;
			for(int col = 0; col<c;col++){
				string word = "";
				set<char> seen;
				for(int row=r-1;row>=0;row--){
					if(seen.find(a[row][col])==seen.end())
						word+=a[row][col];
					seen.insert(a[row][col]);
				}
				words.push_back(word);
			}
			printOrder(words,words.size(),26);
			cout<<endl;
		}
	}
}