import java.io.*; import java.util.*; import java.util.function.*; import java.util.stream.*; public class Main { public static void main(String[] args) throws Exception { Scanner sc = new Scanner(); int n = sc.nextInt(); Map counts = new HashMap<>(); for (int i = 0; i < n; i++) { String s = sc.next(); for (int j = 0; j < 26; j++) { String x = add(s, (char)(j + 'a')); counts.put(x, counts.getOrDefault(x, 0) + 1); } } System.out.println(counts.keySet().stream().filter(x -> counts.get(x) == 1).findFirst().orElse("-1")); } static String add(String s, char x) { char[] arr = (s + x).toCharArray(); Arrays.sort(arr); return new String(arr); } } class Scanner { BufferedReader br; StringTokenizer st = new StringTokenizer(""); StringBuilder sb = new StringBuilder(); public Scanner() { try { br = new BufferedReader(new InputStreamReader(System.in)); } catch (Exception e) { } } public int nextInt() { return Integer.parseInt(next()); } public long nextLong() { return Long.parseLong(next()); } public double nextDouble() { return Double.parseDouble(next()); } public String next() { try { while (!st.hasMoreTokens()) { st = new StringTokenizer(br.readLine()); } } catch (Exception e) { e.printStackTrace(); } finally { return st.nextToken(); } } }