#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
template<class... T> void dbg(T... x) { char e{}; ((cerr << e << x, e = ' '), ...); }
template<class T> void org(T l, T r) { while (l != r) cerr << ' ' << *l++; cerr << '\n'; }
#define debug(x...) dbg(#x, '=', x, '\n')
#define olist(x...) dbg(#x, '='), org(x)
#else
#define debug(...) ((void)0)
#define olist(...) ((void)0)
#endif
#define all(v) (v).begin(), (v).end()
#define rall(v) (v).rbegin(), (v).rend()
#define ff first
#define ss second

using u32 = unsigned int;
using i64 = long long;
using u64 = unsigned long long;
using i128 = __int128;
using u128 = unsigned __int128;

template<class T>
inline constexpr T inf = numeric_limits<T>::max() / 2;
constexpr int mod = 998244353;

template<class T> bool chmin(T &a, T b) { return (b < a and (a = b, true)); }
template<class T> bool chmax(T &a, T b) { return (a < b and (a = b, true)); }
template<class... T> int add(T... x) { int t{}; return (((t += x) %= mod), ...), t; }
template<class... T> int mul(T... x) { i64 t{1}; return (((t *= x) %= mod), ...), t; }

void solve() {
    int n, m;
    cin >> n >> m;

    vector<int> pos(n);
    vector<string> S(n);
    for (int i = 0; i < n; i++) {
        cin >> pos[i] >> S[i];
        pos[i]--;
    }

    auto check = [&](int s) -> string {
        string str(m, '?');
        for (int i = s; i < n; i++) {
            for (int j = 0; j < S[i].size(); j++) {
                if (str[pos[i] + j] == '?') str[pos[i] + j] = S[i][j];
                else if (str[pos[i] + j] != S[i][j]) {
                    return "-";
                }
            }
        }
        return str;
    };

    bool bad = false;
    string ans;

    if (string t = check(0); t != "-" and t.find('?') == string::npos) {
        ans = t;
    }
    
    for (int i = 0; i < n; i++) {
        swap(pos[i], pos[0]);
        swap(S[i], S[0]);
        string t = check(1);
        debug(i, t);
        if (t != "-" and t.find('?') == string::npos) {
            if (ans.empty()) {
                ans = t;
            } else if (ans != t) {
                bad = true;
            }
        }
        swap(pos[i], pos[0]);
        swap(S[i], S[0]);
    }

    if (bad) {
        cout << "-2\n";
    } else if (ans.empty()) {
        cout << "-1\n";
    } else {
        cout << ans << '\n';
    }
}

signed main() {
    cin.tie(0)->sync_with_stdio(false);
    cin.exceptions(cin.failbit);
    int T = 1;
    // cin >> T;
    while (T--) {
        solve();
    }
    
    return 0;
}