func_code_string: #include <bits/stdc++.h>   using namespace std;   char a[20][20], ans[20];   int n, m;   bool isValid() {     for (int i = 2; i <= n; i++) {       int c = 0;       for (int j = 1; j <= m; j++) c += (ans[j] != a[i][j]);       if (c > 1) return false;     }     return true;   }   void fun() {     cin >> n >> m;     for (int i = 1; i <= n; i++) cin >> (a[i] + 1);     for (int i = 1; i <= m; i++) ans[i] = a[1][i];     ans[m + 1] =   0 ;     if (isValid())       cout << (ans + 1) <<   n ;     else {       bool isGood = false;       for (int i = 1; i <= m && !isGood; i++) {         for (char c =  a ; c <=  z  && !isGood; c++) {           if (c != a[1][i]) {             ans[i] = c;             if (isValid()) {               cout << (ans + 1) <<   n ;               isGood = true;             }           }         }         ans[i] = a[1][i];       }       if (!isGood) cout <<  -1 n ;     }   }   int main() {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     cout.tie(NULL);     int t;     cin >> t;     while (t--) {       fun();     }     return 0;   }   