#include<bits/stdc++.h>
using namespace std;
#define Fopen freopen("_in.txt","r",stdin); freopen("_out.txt","w",stdout);
#define LL long long
#define ULL unsigned LL
#define fi first
#define se second
#define pb push_back
#define lson l,m,rt<<1
#define rson m+1,r,rt<<1|1
#define lch(x) tr[x].son[0]
#define rch(x) tr[x].son[1]
#define max3(a,b,c) max(a,max(b,c))
#define min3(a,b,c) min(a,min(b,c))
typedef pair<int,int> pll;
const int inf = 0x3f3f3f3f;
const int _inf = 0xc0c0c0c0;
const LL INF = 0x3f3f3f3f3f3f3f3f;
const LL _INF = 0xc0c0c0c0c0c0c0c0;
const LL mod =  (int)1e9+7;
const int N = 3e4;
string s[N];
int dif[N];
int num[26], num2[26];
int main(){
    int k, n;
    scanf("%d%d", &k, &n);
    for(int i = 1; i <= k; ++i)
        cin >> s[i];
    int f = 0;
    for(int i = 0; i < n; ++i)
        ++num[s[1][i]-‘a‘];
    for(int i = 0; i < 26; ++i)
        f += (num[i] > 1);
    for(int i = 2; i <= k; ++i){
        for(int j = 0; j < n; ++j){
            ++num2[s[i][j]-‘a‘];
            dif[i] += (s[i][j] != s[1][j]);
        }
        for(int j = 0; j < 26; ++j){
            if(num[j] != num2[j]){
                puts("-1");
                return 0;
            }
            num2[j] = 0;
        }
    }
    for(int i = 0; i < n; ++i){
        for(int j = i+1; j < n; ++j){
            int ok = 1;
            for(int z = 2; z <= k; ++z){
                int cnt = dif[z];
                cnt -= (s[1][i] != s[z][i]);
                cnt -= (s[1][j] != s[z][j]);
                cnt += (s[1][j] != s[z][i]);
                cnt += (s[1][i] != s[z][j]);
                if((!cnt && f) || cnt == 2) ok++;
            }
            if(ok == k){
                swap(s[1][i], s[1][j]);
                cout << s[1] << endl;
                return 0;
            }
        }
    }
    puts("-1");
    return 0;
}