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/*
 * To change this template, choose Tools | Templates
 * and open the template in the editor.
 */
//package test;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.HashMap;
import java.util.StringTokenizer;

/**
 *
 * @author Admin
 */
public class Test {

    private static String a[];
    private static BufferedReader in;
    private static StringTokenizer tok;
    private static PrintWriter out;
    private static String ans = "zzzzzzzzzzzzzzzzzzzzzz";

    private static String readToken() throws IOException {
        while (tok == null || !tok.hasMoreTokens()) {
            tok = new StringTokenizer(in.readLine());
        }
        return tok.nextToken();
    }

    private static int readInt() throws IOException {
        return Integer.parseInt(readToken());
    }

    private static String readString() throws IOException {
        return readToken();
    }

//    private static void solve(String str, HashMap<String, Integer> map) {
//        if (ans == null) {
//            for (int i = 0; i < 26; i++) {
//                if (!map.containsKey(str + (char) ('a' + i))) {
//                    ans = str + (char) ('a' + i);
//                    i = 26;
//                }
//            }
//            if(ans == null){
//                for(int i = 0; i < 26; i++){
//                    if(ans == null) solve(str + (char) ('a' + i), map);
//                }
//            }
//        }
//    }
    private static void solve(String str, HashMap<String, Integer> map) {
        for (int i = 0; i < 26; i++) {
            if (!map.containsKey(str + (char) ('a' + i))) {
                if (ans.compareTo(str + (char) ('a' + i)) > 0) {
                    ans = str + (char) ('a' + i);
                    i = 26;
                }
            }
        }
        for (int i = 0; i < 26; i++) {
            if (ans == null) {
                solve(str + (char) ('a' + i), map);
            }
        }
    }

    public static void main(String[] args) throws IOException {
        in = new BufferedReader(new InputStreamReader(System.in));
        out = new PrintWriter(System.out);
        int n = readInt();
        a = new String[n];
        for (int i = 0; i < n; i++) {
            a[i] = readString();
        }
        HashMap<String, Integer> map = new HashMap<>();
        for (int i = 0; i < n; i++) {
            int len = 1;
            while (len <= a[i].length()) {
                for (int j = 0; j < a[i].length(); j++) {
                    if (j + len <= a[i].length()) {
                        String str = a[i].substring(j, j + len);
                        if (!map.containsKey(str)) {
                            map.put(str, 0);
                        }
                    }
                }
                len++;
            }
        }
        solve("", map);
        out.write(ans);
        in.close();
        out.close();

    }
}
