#include <bits/stdc++.h>
#include <atcoder/all>
using namespace std;
using namespace atcoder;
#define rep(i, n) for(int i=0;i<(n);++i)
#define rep1(i, n) for(int i=1;i<=(n);i++)
#define ll long long
using mint = modint998244353;
using P = pair<ll,ll>;
using lb = long double;
using T = tuple<ll, ll, ll>;
#ifdef LOCAL
#  include <debug_print.hpp>
#  define dbg(...) debug_print::multi_print(#__VA_ARGS__, __VA_ARGS__)
#else
#  define dbg(...) (static_cast<void>(0))
#endif

int main()
{
    int n;
    cin >> n;
    vector<string> s(n);
    rep(i,n) cin >> s[i];
    unordered_map<string, int> st;
    rep(i,n)rep(j,26) {
        string t = s[i];
        t += 'a' + j;
        sort(t.begin(),t.end());
        st[t]++;
    }
    for(auto p : st) {
        if(p.second==1) {
            cout << p.first << endl;
            return 0;
        }
    }
    cout<<-1<<endl;
    return 0;
}