#include <bits/stdc++.h>
using namespace std;
#define fori(x, y) for (int i=(x);i<=(y);++i)
#define forj(x, y) for (int j=(x);j<=(y);++j)
#define fork(x, y) for (int k=(x);k<=(y);++k)

typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

#define debug(x) cerr << #x << " = " << x << endl

const int N = 105;

char str[N];
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    int n, m;
    cin >> n >> m;
    vector<pair<int,string>> vec;
    fori(1, n) {
        int x;
        string str;
        cin >> x >> str;
        vec.push_back({x, str});
    }
    
    set<string> ans;
    fori(0, n-1) {
        forj(1, m) str[j] = 0;
        bool can = true;
        forj(0, n-1) {
            if (i == j) continue;
            fork(0, vec[j].second.length()-1) {
                int id = vec[j].first;
                if (str[id+k] != 0 && str[id+k] != vec[j].second[k]) {
                    can = false;
                    break;
                }
                str[id+k] = vec[j].second[k];
            }
            if (!can) break;
        }
        forj(1, m) if (str[j] == 0) {
            can = false;
            break;
        }
        if (can) {
            ans.insert(string(str+1));
        }
    }
    // debug(val);
    forj(1, m) str[j] = 0;
    bool can = true;
    forj(0, n-1) {
        fork(0, vec[j].second.length()-1) {
            int id = vec[j].first;
            if (str[id+k] != 0 && str[id+k] != vec[j].second[k]) {
                can = false;
                break;
            }
            str[id+k] = vec[j].second[k];
        }
        if (!can) break;
    }
    forj(1, m) if (str[j] == 0) {
        can = false;
        break;
    }
    if (can) {
        ans.insert(string(str+1));
    }
    if (ans.size() == 0) {
        cout << -1 << endl;
    }
    else if (ans.size() > 1) {
        cout << -2 << endl;
    }
    else {
        cout << *ans.begin() << endl;
    }

    
    return 0;
}