3 4
kbub
kbbu
ubkb
 5 5
zbibx
zbbix
zbibx
xbibz
xbibz
 100 2
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
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qz
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qz
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qz
qz
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qz
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qz
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qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
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qz
qz
qz
qz
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qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
qz
 100 3
zsk
zsk
zsk
zsk
skz
skz
kzs
skz
kzs
kzs
zsk
kzs
kzs
kzs
skz
kzs
skz
skz
kzs
zsk
kzs
kzs
kzs
skz
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skz
zsk
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kzs
skz
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kzs
zsk
zsk
kzs
skz
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kzs
kzs
skz
kzs
kzs
skz
kzs
skz
kzs
kzs
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kzs
kzs
kzs
kzs
skz
kzs
skz
skz
skz
skz
zsk
kzs
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zsk
kzs
skz
skz
skz
kzs
kzs
skz
skz
kzs
kzs
skz
zsk
skz
zsk
kzs
kzs
skz
kzs
kzs
kzs
zsk
kzs
kzs
zsk
kzs
skz
zsk
skz
skz
skz
kzs
zsk
kzs
kzs
skz
 5 5
eellh
ehlle
ehlle
helle
hlele
 2 3
ghn
ghn
 2 4
ayax
abac
 2 6
abcdef
badcef
 1 2
yu
 2 4
ajax
azad
 2 2
nm
nm
 2 2
ac
ca
 5 5
gyvnn
gnvny
vygnn
gynvn
gnvny
 2 2
ab
cd
 3 3
uvh
vhu
huv
 3 2
xh
xh
xh
 3 2
cg
cg
cg
 8 23
mgiomcytqdvoihhcirldmuj
mgmoicytqdvoihhicrldmuj
mgmomcytqdvoihhicrldiuj
mgcomcytqdvoihhiirldmuj
mgiimcytqdvoihhocrldmuj
mgioicytqdvoihhmcrldmuj
mgiomcytqdvodhhicrlimuj
mgiomcytjdvoihhicrldmuq
 8 6
mnionk
mnikno
inmkno
mnnkio
oniknm
oniknm
mkinno
mnikon
 1 3
vts
 2 3
abc
abz
 2 4
abcd
ccdc
 2 5
dbcag
acbdh
 2 16
hmlqgaepsgpdbzyk
chhhburuvnyirrim

kbub
 zbibx
 zq
 szk
 helle
 hgn
 -1
 bacdef
 uy
 -1
 mn
 -1
 gyvnn
 -1
 vuh
 hx
 gc
 mgiomcytqdvoihhicrldmuj
 mnikno
 tvs
 -1
 -1
 -1
 -1

#pragma GCC optimize("Ofast")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,fma,abm,mmx,avx,tune=native")
#include <bits/stdc++.h>
#include <ext/pb_ds/assoc_container.hpp> // Common file  
#include <ext/pb_ds/tree_policy.hpp>  
#include <functional> // for less  
#include <iostream>  
using namespace __gnu_pbds;  
using namespace std;
#define ll unsigned long long
typedef tree<pair<ll,ll>, null_type, less<pair<ll,ll>>, rb_tree_tag,tree_order_statistics_node_update>  ordered_set;
using namespace std;
 
#define fr(i,a,b) for (int i = (a), _b = (b); i <= _b; i++)
#define frr(i,a,b) for (int i = (a), _b = (b); i >= _b; i--)
#define rep(i,n) for (long long i = 0, _n = (n); i < _n; i++)
#define repr(i,n) for (long long i = (n) - 1; i >= 0; i--)
#define foreach(it,ar) for ( typeof(ar.begin()) it = ar.begin(); it != ar.end(); it++ )
#define fill(ar,val) memset(ar, val, sizeof(ar))
#define fill0(ar) fill((ar), 0)
#define fillinf(ar, n) fr(i,0,(n)) ar[i] = INF
#define debug(x) cout<<#x<<": "<<x<<endl
#define arr1d(a,n) cout << #a << " : "; fr(_,1,n) cout << a[_] << ' '; cout << endl;
#define arr1d0(a,n) cout << #a << " : "; rep(_,n) cout << a[_] << ' '; cout << endl;
#define arr2d(a,n,m) cout << #a << " :" << endl; fr(_,1,n) {fr(__,1,m) cout << a[_][__] << ' '; cout << endl;}
#define arr2d0(a,n,m) cout << #a << " :" << endl; rep(_,n) {rep(__,m) cout << a[_][__] << ' '; cout << endl;}
/*Author Ritick Goenka || ritick(codechef) ||ritick(codeforces) */
/*IIT Roorkee = <3 */
#define ull unsigned long long
#define ld long double
#define ui unsigned int
#define all(ar) ar.begin(), ar.end()
#define pb push_back
#define mp make_pair
#define ff first
#define ss second
#define y0 yyyyyy0
auto clk=clock();
#define y1 yyyyyy1
#define setb __builtin_popcount
#define BIT(n) (1<<(n))
#define SQR(x) ((x) * (x))
#define CUBE(x) ((x) * (x) * (x))
#define LSOne(S) (S) & (-S)
inline bool EQ(double a, double b) {return fabs(a - b) < 1e-9;}
 
typedef pair<int, int> ii;
typedef pair<int, ii> iii;
typedef vector<ii> vii;
typedef vector<int> vi;
typedef vector<string> vs;
 
template<typename T>inline T gcd(T a, T b){if (b == 0)return a;else return gcd(b, a % b);}
// template<typename T>inline T lcm(T a, T b){return (a * b) / gcd(a, b);}
template<typename T> string toStr(T x) {stringstream st; st << x; string s; st >> s; return s;}
template<class T>
void splitStr(const string &s, vector<T> &out)
{
    istringstream in(s);
    out.clear();
    copy(istream_iterator<T>(in), istream_iterator<T>(), back_inserter(out));
}
 
inline int two(int n) {return 1 << n;}
inline int isOnBit(int n, int b) {return (n >> b) & 1;}
inline void onBit(int & n, int b) {n |= two(b);}
inline void offBit(int & n, int b) {n &= ~two(b);}
inline int lastBit(int n) {return n & (-n);}
inline int cntBit(int n) {int res = 0; while(n && ++res) n -= n &(-n); return res;}
 
const int dx4[] = {-1, 0, 1, 0};
const int dy4[] = {0, 1, 0, -1};
const int dx8[] = {-1, 0, 1, 0, -1, -1, 1, 1};
const int dy8[] = {0, 1, 0, -1, -1, 1, -1, 1};
 
#define INP "test.inp"
#define OUT "test.out"
 
#define PI 3.1415926535897932385
#define INF 10000000000000000ll
#define EPS 1e-7
#define MAXN 100000
#define MO 1000000007
#define dec decr
#define endl '\n'
 
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
ll gcd(ll a, ll b) 
{ 
    if (a == 0) 
        return b; 
    return gcd(b % a, a); 
} 
ll modexp(ll x, ll y, ll p)
{
    ll res = 1;   
    x = x % p;  
    while (y > 0)
    {
        if (y %2==1)    
            res = (res*x) % p;
        y = y>>1;     
        x = (x*x) % p; 
    }
    return res;
}


int main () 
{
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    cout<<fixed;
    cout<<setprecision(9); 
    #ifdef rg
    freopen("input.txt", "r", stdin);
    freopen("output.txt", "w", stdout);
    #endif
    ll n,m;
    cin>>n>>m;
    string st[n];
    rep(i,n)
        cin>>st[i];
    vector<char>v;
    vector<int>v1;
    string s1=st[0];
    string s2;
    for(ll i=1;i<n;i++)
    {
        if(s1!=st[i])
        {
            s2=st[i];
            break;
        }
        if(i==n-1)
        {
            swap(s1[0],s1[1]);
            cout<<s1;
            return 0;
        }
    }
    ll ar[26]={0};
    rep(i,m)
    {
        ar[s1[i]-'a']++;
    }
    rep(i,n)
    {
        ll chk[26]={0};
        rep(j,m)
        {
            chk[st[i][j]-'a']++;
        }
        rep(j,26)
        {
            if(chk[j]!=ar[j])
            {
                cout<<"-1";return 0;
            }
        }
    }
    rep(i,m)
    {
        if(s1[i]!=s2[i])
        {
            v.pb(s1[i]);
            v1.pb(i);
        }
    }
    if(v.size()>4)
    {
        cout<<"-1";
        return 0;
    }
    do
    {   
        bool fl=1;
        rep(i,v1.size())
        {
            s1[v1[i]]=v[i];
        }
        rep(i,n)
        {
            ll x=0;
            rep(j,m)
            {
                if(s1[j]!=st[i][j])
                {
                    x++;
                }
            }
            if(x>2)
            {
                fl=0;
                break;
            }
        }
        if(fl)
        {
            cout<<s1;
            return 0;
        }
    }while(next_permutation(v1.begin(),v1.end()));
    cout<<"-1";
 
    #ifdef rg
    cout<<endl<<endl<<"Time elapsed: "<<(double)(clock()-clk)/CLOCKS_PER_SEC<<endl;
    #endif
    return 0;
}   