#include <bits/stdc++.h>
using namespace std;

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int n, k;
    if (!(cin >> n >> k)) return 0;
    vector<string> words(n);
    for (int i = 0; i < n; i++) cin >> words[i];
    vector<vector<int>> adj(k);
    vector<int> in_degree(k, 0);

    // We only care about characters 0 to k-1 (e.g. 'a' to 'a'+k-1)
    bool possible = true;
    for (int i = 0; i < n - 1; i++) {
        string w1 = words[i];
        string w2 = words[i+1];
        int len = min(w1.length(), w2.length());
        bool found = false;
        for (int j = 0; j < len; j++) {
            if (w1[j] != w2[j]) {
                int u = w1[j] - 'a';
                int v = w2[j] - 'a';
                if (u < k && v < k) {
                    adj[u].push_back(v);
                    in_degree[v]++;
                }
                found = true;
                break;
            }
        }
        if (!found && w1.length() > w2.length()) {
            possible = false; // invalid sort e.g. "apple" before "app"
        }
    }
    if (!possible) {
        cout << -1 << "\n";
        return 0;
    }
    queue<int> q;
    for (int i = 0; i < k; i++) {
        if (in_degree[i] == 0) q.push(i);
    }
    string result = "";
    while (!q.empty()) {
        int u = q.front();
        q.pop();
        result += (char)('a' + u);
        for (int v : adj[u]) {
            in_degree[v]--;
            if (in_degree[v] == 0) q.push(v);
        }
    }
    if (result.length() == k) {
        cout << result << "\n";
    } else {
        cout << -1 << "\n";
    }
    return 0;
}