#include <bits/stdc++.h>   using namespace std;   string st[5050];   int fr[27];   int main() {     memset(fr, 0, sizeof(fr));     ios::sync_with_stdio(0);     cin.tie(0);     cout.tie(0);     int n, k;     cin >> k >> n;     for (int i = 0; i < k; i++) {       cin >> st[i];     }     string res = st[0];     memset(fr, 0, sizeof(fr));     bool du = false;     for (int i = 0; i < n; i++) {       fr[res[i] -  a ]++;       if (fr[res[i] -  a ] > 1) {         du = true;       }     }     int ma = 0;     int mi = 0;     bool po = true;     for (int i = 1; i < k; i++) {       int df = 0;       int ct[27] = {0};       for (int j = 0; j < n; j++) {         ct[st[i][j] -  a ]++;         if (st[i][j] != res[j]) {           df++;         }       }       if (df > 4) {         po = false;         break;       }       for (int j = 0; j < 27; j++) {         if (ct[j] != fr[j]) {           po = false;           break;         }       }       if (df > ma) {         ma = df;         mi = i;       }     }     if (!po) {       cout << -1 <<   n ;       return 0;     }     if (ma == 0) {       swap(res[0], res[1]);       cout << res <<   n ;       return 0;     }     if (ma == 2) {       if (!du) {         cout << -1 <<   n ;         return 0;         cout << -1 <<   n ;         return 0;       }       cout << res <<   n ;       return 0;     }     int q[4];     int ctr = 0;     for (int i = 0; i < n; i++) {       if (res[i] != st[mi][i]) {         q[ctr++] = i;       }     }     po = false;     int tr[2] = {0, 0};     for (int id = 0; id < 3; id++) {       if (ma < 4) {         q[3] = q[id];       } else if (id > 0)         break;       for (int i = 0; i < 4; i++) {         for (int j = 0; j < 4; j++) {           if (po) break;           int md = 0;           swap(res[q[i]], res[q[j]]);           for (int c = 1; c < k; c++) {             int df = 0;             for (int l = 0; l < n; l++) {               if (res[l] != st[c][l]) {                 df++;               }             }             md = max(md, df);           }           swap(res[q[i]], res[q[j]]);           if (md == 2) {             po = true;             tr[0] = i;             tr[1] = j;           }         }       }     }     if (po) {       swap(res[q[tr[0]]], res[q[tr[1]]]);       cout << res <<   n ;     } else {       cout << -1 <<   n ;     }   }   