#include <bits/stdc++.h>
using namespace std;
#define rep(i, n) for(int i=0; i<n; i++)
#define debug 0
#define YES cout << "Yes" << endl;
#define NO cout << "No" << endl;
using ll = long long;
using ld = long double;
const int mod = 998244353;
const int MOD = 1000000007;
const double pi = atan2(0, -1);
const int inf = 1 << 31 - 1;
const ll INF = 1LL << 63-1;
#include <time.h>
#include <chrono>

//vectorの中身を空白区切りで出力
template<typename T> 
void print1(vector<T> v) {
	for (int i = 0; i < v.size(); i++) {
		cout << v[i];
		if (i < v.size() - 1) {
			cout << " ";
		}
	}
}

//vectorの中身を改行区切りで出力
template<typename T>
void print2(vector<T> v) {
	for (auto x : v) {
		cout << x << endl;
	}
}

//二次元配列を出力
template<typename T>
void printvv(vector<vector<T>> vv) {
	for (vector<T> v : vv) {
		print1(v);
		cout << endl;
	}
}

//vectorを降順にソート
template<typename T>
void rsort(vector<T> &v) {
	sort(v.begin(), v.end());
	reverse(v.begin(), v.end());
}

//昇順priority_queueを召喚
template<typename T>
struct rpriority_queue {
	priority_queue<T, vector<T>, greater<T>> pq;

	void push(T x) {
		pq.push(x);
	}

	void pop() {
		pq.pop();
	}

	T top() {
		return pq.top();
	}

	size_t size() {
		return pq.size();
	}

	bool empty() {
		return pq.empty();
	}
};

int main() {
	int N;
	cin >> N;
	map<string, int>mp;
	rep(i, N) {
		string S;
		cin >> S;
		for (char c = 'a'; c <= 'z'; c++) {
			string T = S + c;
			sort(T.begin(), T.end());
			mp[T]++;
		}
	}

	for (auto p : mp) {
		string T = p.first;
		int k = p.second;
		if (k == 1) {
			cout << T << endl;
			return 0;
		}
	}
	cout << -1 << endl;
}