gistfile1.txt

#pragma GCC optimize("Ofast,no-stack-protector")
#pragma GCC target("avx,tune=native")

#include <bits/stdc++.h>

using namespace std;

typedef long long int ll;
typedef pair<ll, ll> pii;

template<typename T>
using pq_gt = priority_queue<T, vector<T>, greater<T>>;
template<typename T>
using pq_lt = priority_queue<T, vector<T>, less<T>>;

#define se second
#define fi first
#define pb push_back
string grid[2020];
string s;
map<string, int> m;
string ans;
vector<set<char>>v;

bool solve(int a, int b, int c) {
	//cout<<s<<endl;
	if(c==b) {
		//cout<<m[s]<<endl;
		if(m[s]==0) {
			ans = s;
			return true;
		}
		//cout<<"false"<<endl;
		return false;
	}
	for(char ch:v[c]) {
		//cout<<i<<" "<<a<<endl;
		s += ch;
		if(solve(a,b,c+1)) {
			return true;
		}
		s.pop_back();
	}
	return false;
}

int main() {
    ios::sync_with_stdio(false);
    int t; 
   	cin>>t;
   	for(int tt = 1; tt<=t; tt++) {
   		cout<<"Case #"<<tt<<": ";
   		int a,b;
   		m.clear();
   		cin>>a>>b;
   		s = "";
   		v.clear();
   		v.resize(b);
   		for(int i = 0; i < a; ++i) {
   			cin>>grid[i];
   			m[grid[i]]++;
   			for(int j = 0; j < b; ++j) {
   				v[j].insert(grid[i][j]);
   			}
   		}
   		if(solve(a,b, 0)) {
   			cout<<ans<<endl;
   		} else {
   			cout<<"-"<<endl;
   		}
   	}
}