import java.util.*;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		char[][] s = new char[n][];
		for (int i = 0; i < n; i++) {
			s[i] = sc.next().toCharArray();
		}
		sc.close();

		Map<String, Integer> map = new HashMap<>();
		for (int i = 0; i < n; i++) {
			for (char c = 'a'; c <= 'z'; c++) {
				int len = s[i].length;
				char[] t = new char[len + 1];
				System.arraycopy(s[i], 0, t, 0, len);
				t[len] = c;
				Arrays.sort(t);
				String k = String.valueOf(t);
				map.put(k, map.getOrDefault(k, 0) + 1);
			}
		}
		for (String k : map.keySet()) {
			if (map.get(k) == 1) {
				System.out.println(k);
				return;
			}
		}
		System.out.println(-1);
	}
}