func_code_string: #include <bits/stdc++.h>   using namespace std;   int T;   string hxt[16];   int main() {     cin >> T;     while (T--) {       int n, m;       cin >> n >> m;       for (int i = 1; i <= n; i++) cin >> hxt[i];       bool iif = false;       for (int i = 0; i < m; i++) {         char x = hxt[1][i];         for (char j =  a ; j <=  z ; j++) {           hxt[1][i] = j;           bool flag = true;           for (int k = 2; k <= n; k++) {             int js = 0;             for (int l = 0; l < m; l++) {               if (hxt[k][l] != hxt[1][l]) js++;               if (js > 1) break;             }             if (js > 1) {               flag = false;               break;             }           }           if (flag) {             cout << hxt[1] << endl;             iif = true;             break;           }           hxt[1][i] = x;         }         if (iif) break;       }       if (!iif) cout << -1 << endl;     }     return 0;   }   