func_code_string: #include <bits/stdc++.h>   using namespace std;   using ll = long long;   const int INF = 2e9;   vector<string> v;   int n, m;   int diff(string a, string b) {     int i, cc = 0;     for (i = 0; i < m; ++i) cc += (a[i] != b[i]);     return cc;   }   string solve() {     string s = v[0], t;     int i, j, cc;     char c;     bool f;     for (i = 0; i < m; ++i) {       t = s;       for (c =  a ; c <=  z ; ++c) {         t[i] = c;         cc = 0;         f = 1;         for (j = 0; j < n; ++j) {           cc = diff(t, v[j]);           if (cc > 1) f = 0;         }         if (f) return t;       }     }     return  -1 ;   }   int main() {     ios_base::sync_with_stdio(false);     cin.tie(NULL);     cout.tie(NULL);     int tc, ti = 1;     cin >> tc;     while (tc--) {       int i, j;       cin >> n >> m;       v.resize(n);       for (i = 0; i < n; ++i) cin >> v[i];       cout << solve() <<   n ;     }     return 0;   }   