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();
        String[] base = new String[n];
        int[][] counts = new int[n][26];
        for (int i = 0; i < n; i++) {
            base[i] = sc.next();
            for (int c : base[i].toCharArray()) {
                counts[i][c - 'a']++;
            }
        }
        Map<MyString, Integer> exists = new HashMap<>();
        for (int i = 0; i < n; i++) {
            for (int j = 0; j < 26; j++) {
                counts[i][j]++;
                MyString x = new MyString(counts[i]);
                exists.put(x, exists.getOrDefault(x, 0) + 1);
                counts[i][j]--;
            }
        }
        for (MyString x : exists.keySet()) {
            if (exists.get(x) == 1) {
                System.out.println(x);
                return;
            }
        }
        System.out.println(-1);
    }
    
    static class MyString {
        String s;
        
        public MyString(int[] counts) {
            StringBuilder sb = new StringBuilder();
            for (int i = 0; i < 26; i++) {
                for (int j = 0; j < counts[i]; j++) {
                    sb.append((char)(i + 'a'));;
                }
            }
            s = sb.toString();
        }
        
        public int hashCode() {
            return s.hashCode();
        }
        
        public boolean equals(Object o) {
            MyString x = (MyString)o;
            return s.equals(x.s);
        }
        
        public String toString() {
            return s;
        }
    }
}
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();
        }
    }
    
}