#include <bits/stdc++.h>
using namespace std;
#define all(v) (v).begin(),(v).end()
#define pb(a) push_back(a)
#define rep(i, n) for(int i=0;i<n;i++)
#define foa(e, v) for(auto&& e : v)
using ll = long long;
const ll MOD7 = 1000000007, MOD998 = 998244353, INF = (1LL << 60);
#define dout(a) cout<<fixed<<setprecision(10)<<a<<endl;

int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    int n;
    cin >> n;
    vector<string> s(n);
    rep(i, n) {
        cin >> s[i];
    }
    string t = "";
    map<string, int> mp;
    rep(i, n) {
        string u = s[i];
        rep(j, 26) {
            string S = u + char('a' + j);
            sort(all(S));
            mp[S] ++;
        }
    }
    for(auto [x, cnt] : mp) {
        if(cnt == 1) {
            cout << x << endl;
            return 0;
        }
    }
    cout << -1 << endl;
    return 0;
}