func_code_string: #include <bits/stdc++.h>   using namespace std;   int main() {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     cout.tie(NULL);     long long t;     cin >> t;     while (t--) {       int n, m;       cin >> n >> m;       vector<string> v(n);       for (int i = 0; i < n; i++) cin >> v[i];       int c = 0;       string ans =  -1 , temp;       for (int i = 0; i < m; i++) {         temp = v[0];         for (char j =  a ; j <=  z ; j++) {           temp[i] = j;           for (int k = 0; k < n; k++) {             c = 0;             for (int l = 0; l < m; l++)               if (v[k][l] != temp[l]) c++;             if (c > 1) break;           }           if (c <= 1) {             ans = temp;             break;           }         }         if (ans !=  -1 ) break;       }       cout << ans <<   n ;     }     return 0;   }   