//#pragma GCC optimize ("O3")
//#pragma GCC target ("sse4")
//#incluse <bits/stdc++.h>
#include <iostream>
#include <sstream>
#include <fstream>
#include <string>
#include <cstring>
#include <vector>
#include <deque>
#include <queue>
#include <stack>
#include <set>
#include <cstring>
#include <cassert>
#include <list>
#include <tuple>
#include <map>
#include <unordered_map>
#include <iomanip>
#include <algorithm>
#include <functional>
#include <cstdlib>
#include <cstdio>
#include <random>
using namespace std;

// ************ Setting up  ************
#define FPTU ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
#define TIME  (1.0 * clock() / CLOCKS_PER_SEC)
#define BUG(x) {cerr << #x << " = " << x << endl;}
#define pii pair<int,int>
#define pip pair<int,pii>
#define ppi pair<pii,int>
#define ll  long long
#define ull unsigned long long
#define usi unsigned int
#define pll pair<ll,ll>
#define plp pair<ll,pll>
#define ppl pair<pll,ll>
#define ppp pair<pii,pii>
#define oo 1000111000
#define ooo 1000111000111000111
#define inf 0x3f //4557430888798830399
#define fi first
#define se second
#define vt vector
#define pb push_back
#define all(arr) arr.begin(),arr.end()
#define bit(n, i) (((n) >> (i)) & 1)
#define db(x) cerr << #x << " = " << (x) << '\n';

ll rand_num(ll a,ll b) {
    random_device rd;
    mt19937 mt(rd());
    uniform_int_distribution<ll> dist(a,b);
    return dist(mt);
}

int mod=1e9+7; // MODDDDDDDDDDDDD

template <typename T> void minimize(T &a, T b){ if(a>b)  a=b; }
template <typename T> void maximize(T &a, T b){ if(a<b)  a=b; }
template <typename T> void add(T &a, T b){ a+=b; if(a>=mod) a-=mod; }
template <typename T> T gcd(T a, T b){
    while(a!=0&&b!=0) if(a>b) a%=b; else b%=a; return a+b; }
void read_file()
{
    freopen("sample.inp","r",stdin);
    freopen("sample.out","w",stdout);
}
// =========> <3 VietHai1709 <3  <=========
#define node pair<int,string>
bool change(string &s,node a){
    string tmp=a.se;
    int st=a.fi;
    for(int i=0;i<(int)tmp.size();i++){
        if(s[st+i]==tmp[i]) continue;
        if(s[st+i]=='#') s[st+i]=tmp[i];
        else return false;
    }
    return true;
}
void Minnnnnnn()
{
    int n,m;
    cin>>n>>m;
    vt<node> a;
    for(int i=1;i<=n;i++){
        int x;
        string t;
        cin>>x>>t;
        a.pb({x-1,t});
    }
    int cnt=0;
    string ans="";
    string tmp;
    for(int i=1;i<=m;i++) tmp+='#';
    
    ans=tmp;
    bool ok=1;
    for(int i=0;i<n;i++){
        if(!change(ans,a[i])){
            ok=0;
            break;
        }
    }
    for(int i=0;i<m;i++) if(ans[i]=='#') ok=0;
    if(ok){
        cout<<ans;
        return;
    }
    
    ans="";
    for(int cur=0;cur<n;cur++){
        string s=tmp;
        bool pos=1;
        for(int i=0;i<n;i++) if(i!=cur){
            if(!change(s,a[i])){
                pos=0;
                break;
            }
        
        }
        // check s again
        for(int i=0;i<m;i++) if(s[i]=='#'){
            pos=0;
            break;
        }
        
        if(pos){
            cnt++;
            ans=s;
        }
    }
    if(cnt==1) cout<<ans;
    else if(cnt==0) cout<<-1;
    else cout<<-2;
}


int main(){
    
    FPTU; //read_file();
    int __=1;
    //cin>>__;
    for(int _=1;_<=__;_++)
    {
        Minnnnnnn();
    }
    cerr << "Time elapsed: " << TIME << " s.\n";

    return 0;
}