func_code_string: #include <bits/stdc++.h>   using namespace std;   const long long mod = 1e9 + 7;   long long powmod(long long a, long long b, long long p) {     long long res = 1;     a %= p;     while (b) {       if (b & 1) res = res * a % p;       a = a * a % p;       b >>= 1;     }     return res;   }   long long gcd(long long a, long long b) { return b ? gcd(b, a % b) : a; }   void solve() {     int n, m;     cin >> n >> m;     vector<string> v(n);     for (int i = 0; i < n; i++) {       cin >> v[i];     }     string ans = v[0];     for (int j = 0; j < m; j++) {       char save = ans[j];       for (char d =  a ; d <=  z ; d++) {         ans[j] = d;         bool flag = true;         for (int z = 0; z < n; z++) {           int cntErrors = 0;           for (int c = 0; c < m; c++) {             if (v[z][c] != ans[c]) {               cntErrors++;             }           }           if (cntErrors > 1) {             flag = false;             break;           }         }         if (flag) {           cout << ans << endl;           return;         }       }       ans[j] = save;     }     cout <<  -1  << endl;   }   int main() {     int t;     cin >> t;     while (t--) {       solve();     }   }   