3 4
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 5 5
zbibx
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xbibz
 100 2
qz
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qz
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qz
qz
qz
qz
 100 3
zsk
zsk
zsk
zsk
skz
skz
kzs
skz
kzs
kzs
zsk
kzs
kzs
kzs
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zsk
skz
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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

#include<bits/stdc++.h>
#include<ext/pb_ds/assoc_container.hpp>
#include<ext/pb_ds/tree_policy.hpp>
#define rep(i, a, b) for(int i=(a); i<(b); i++)
#define repi(i, a, b) for(int i=(a); i>(b); i--)
#define db(x) (cerr << #x << ": " << (x) << '\n')
#define sync ios_base::sync_with_stdio(false), cin.tie(NULL)
#define tests(t) int t; cin >> t; while(t--)
#define iceil(n, x) (((n) + (x) - 1) / (x))
#define ll long long
#define gcd __gcd
#define pb push_back
#define pf push_front
#define pob pop_back
#define pof pop_front
#define sz size()
#define all(v) (v).begin(), (v).end()
#define uni(v) sort(all(v)); (v).erase(unique(all(v)), (v).end());
#define pii pair<int, int>
#define vi vector<int>
#define vpii vector<pii>
#define vvi vector<vi>
#define fi first
#define se second
#define umap unordered_map
#define uset unordered_set
#define pqueue priority_queue
#define si(a) scanf("%d", &a)
#define sll(a) scanf("%lld", &a)
#define bitcount(x) __builtin_popcount(x)
#define cps CLOCKS_PER_SEC
#define PI acos(-1.0l)
#define EPS 1e-6
#define mod 1000000007
#define MOD 1000000007
#define N 2505
using namespace std;

#define trace(...) __f(#__VA_ARGS__, __VA_ARGS__)
template <typename Arg1>
void __f(const char* name, Arg1&& arg1){
    cerr << name << " : " << arg1 << '\n';
}
template <typename Arg1, typename... Args>
void __f(const char* names, Arg1&& arg1, Args&&... args){
    const char* comma = strchr(names + 1, ',');cerr.write(names, comma - names) << " : " << arg1<<" | ";__f(comma+1, args...);
}

template<typename T>
using minpq = priority_queue<T, vector<T>, greater<T>>;

template<typename T>
using maxpq = priority_queue<T>;

//All indexing is 0-based
using namespace __gnu_pbds;
typedef tree<int, null_type, greater<int>, rb_tree_tag,
             tree_order_statistics_node_update> ordered_set;
//methods: find_by_order(k); & order_of_key(k);
//To make it an ordered_multiset, use pairs of (value, time_of_insertion)
//to distinguish values which are similar.

template<class key, class value, class cmp = std::less<key>>
using ordered_map = tree<key, value, cmp, rb_tree_tag, tree_order_statistics_node_update>;
//ordered_map<int, int> my_map;


//Returns no. of values x for which ceil(n / x) == y (y must be > 1).
inline ll CC(ll n, ll y) { return iceil(n, y-1) - iceil(n, y); }

//Returns no. of values x for which floor(n / x) == y
inline ll FF(ll n, ll y) { return n / y - n / (y+1); }

//a and b are assumed to be taken modulo p
inline int add(int a, int b, int p = mod){ int c = a + b; if(c >= p) c -= p; return c;}
inline int sub(int a, int b, int p = mod){ int c = a - b; if(c < 0) c += p; return c;}
inline int mul(int a, int b, int p = mod){ return (a * 1ll * b) % p;}

int n, k;
string ar[N];
bool hasSame = 0; //String has atleast 2 identical chars

bool check(string t) {
    string temp = t;
    uni(temp);
    hasSame = temp.sz != t.sz;

    for(int i = 0; i < k; i++) {
        string &s = ar[i];
        int cnt = 0;
        for(int j = 0; j < n; j++) cnt += t[j] != s[j];
        if(cnt == 2 || (cnt == 0 && hasSame)) {
            continue;
        }
        return 0;
    }
    return 1;
}

void process(int i, int j) {
    vector<int> pos;
    string a = ar[i], b = ar[j];
    rep(l, 0, n) {
        if(a[l] != b[l]) {
            pos.pb(l);
        }
    }

    if(pos.sz > 4) {
        cout << -1 << '\n'; return;
    }

    for(int i : pos) {
        for(int j = 0; j < n; j++) {
            if(j == i) continue;
            swap(a[i], a[j]);
            //trace(a);
            if(check(a)) {
                cout << a << '\n'; return;
            }
            swap(a[i], a[j]);
        }
    }
    cout << -1 << '\n'; return;
}

main()
{   
    #ifndef ONLINE_JUDGE
        freopen("/home/tarun/Desktop/input.txt", "r", stdin);
    //  freopen("/home/tarun/Desktop/output.txt", "w", stdout);
    #endif
    sync;
    clock_t clk = clock();
    cerr << "I will return...\n";
    
    cin >> k >> n; rep(i, 0, k) cin >> ar[i];
    
    for(int j = 1; j < k; j++) {
        if(ar[j] != ar[0]) {
            process(0, j);
            return 0;
        }
    }

    swap(ar[0][0], ar[0][1]);
    cout << ar[0] << '\n';

    cerr << "...don't you ever hang your head.\n";
    cerr << "Time (in ms): " << double(clock() - clk) * 1000.0 / cps << '\n';
}

//Compile using:
//g++ -o filename.exe --std=c++11 filename.cpp
//Use -D CP for defining a macro CP in the local environment





