#include <iostream>

#include <iostream>

#include <algorithm>

#include <cstdio>

#include <math.h>

#include <set>

#include <map>

#include <queue>

#include <string>

#include <string.h>

#include <bitset>

#define REP(i,a,n) for(int i=a;i<=n;++i)

#define PER(i,a,n) for(int i=n;i>=a;--i)

#define hr putchar(10)

#define pb push_back

#define lc (o<<1)

#define rc (lc|1)

#define mid ((l+r)>>1)

#define ls lc,l,mid

#define rs rc,mid+1,r

#define x first

#define y second

#define io std::ios::sync_with_stdio(false)

#define endl '\n'

#define DB(a) ({REP(__i,1,n) cout<<a[__i]<<' ';hr;})

using namespace std;

typedef long long ll;

typedef pair<int,int> pii;

const int P = 1e9+7, INF = 0x3f3f3f3f;

ll gcd(ll a,ll b) {return b?gcd(b,a%b):a;}

ll qpow(ll a,ll n) {ll r=1%P;for (a%=P;n;a=a*a%P,n>>=1)if(n&1)r=r*a%P;return r;}

ll inv(ll x){return x<=1?1:inv(P%x)*(P-P/x)%P;}

inline int rd() {int x=0;char p=getchar();while(p<'0'||p>'9')p=getchar();while(p>='0'&&p<='9')x=x*10+p-'0',p=getchar();return x;}

//head

#ifdef ONLINE_JUDGE

const int N = 1e6+10;

#else

const int N = 111;

#endif

string s[N];

int n, k, vis[N], d[N];

map<int,int> q[N];

int main() {

	cin>>k>>n;

	REP(i,1,k) {

		cin>>s[i];

		for (auto t:s[i]) if (++q[i][t]>1) vis[i]=1;

		if (q[i]!=q[1]) return puts("-1"),0;

	}

	if (k==1) {

		swap(s[1][0],s[1][1]);

		cout<<s[1]<<endl;

		return 0;

	}

	REP(i,2,k) {

		REP(j,0,n-1) d[i]+=s[1][j]!=s[i][j];

	}

	REP(i,0,n-1) REP(j,i+1,n-1) {

		int ok = 1;

		REP(kk,2,k) {

			int t = d[kk];

			d[kk] -= s[1][i]!=s[kk][i];

			d[kk] -= s[1][j]!=s[kk][j];

			d[kk] += s[1][j]!=s[kk][i];

			d[kk] += s[1][i]!=s[kk][j];

			if (d[kk]==1) ok = 0;

			else if (d[kk]==0&&!vis[kk]) ok = 0;

			else if (d[kk]>2) ok = 0;

			d[kk] = t;

		}

		if (ok) {

			swap(s[1][i],s[1][j]);

			return cout<<s[1]<<endl,0;

		}

	}

	puts("-1");

}