#include <bits/stdc++.h>
using namespace std;
const long long N = 1e5 + 5;
bool match(string s1, string s2) {
  long long cnt = 0;
  for (long long i = 0; s1[i]; ++i) {
    if (s1[i] != s2[i]) ++cnt;
    if (cnt > 1) return 0;
  }
  return 1;
}
int32_t main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  cout.tie(0);
  long long t;
  cin >> t;
  while (t--) {
    long long n, m;
    cin >> n >> m;
    vector<string> v(n);
    for (long long i = 0; i < n; ++i) cin >> v[i];
    bool ans = 0;
    string str;
    for (long long i = 0; i < m; ++i) {
      string s = v[0];
      for (long long j = 1; j < n; ++j) {
        s[i] = v[j][i];
        bool all_match = 1;
        for (long long k = 1; k < n; ++k) {
          if (!match(s, v[k])) {
            all_match = 0;
            break;
          }
        }
        if (all_match) {
          ans = 1;
          str = s;
          break;
        }
      }
      if (ans) break;
    }
    if (ans)
      cout << str << "\n";
    else {
      bool all_match = 1;
      for (long long i = 1; i < n; ++i) {
        if (!match(v[0], v[i])) {
          all_match = 0;
          break;
        }
      }
      if (all_match)
        cout << v[0] << "\n";
      else
        cout << -1 << "\n";
    }
  }
}
