3
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 1
z
 30
b
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f
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k
p
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a
c
w
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z
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 5
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 2
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 5
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abcdefghijklmnopqrst
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 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
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ddfhdfhlasjt
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 1
epkotfpkkrhhmuipmtdk
 10
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 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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xpbtbpftimnn
khbknqbvdkdd
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skrvnyz
avptgpjmenfzfxwckmt
 30
e
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ge
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t
bq
bw
 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

import java.util.Arrays;
import java.util.Scanner;


public class NewProblem {

    static boolean[] charOccurances = new boolean[26];
    static String[] words;
    /**
     * @param args
     */
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int numWords = in.nextInt();
        in.nextLine();
        words = new String[numWords];
        for(int i = 0; i < numWords; i++)
            words[i] = in.nextLine();
        Arrays.sort(words);
        findOccurances();
        boolean foundSolution = false;
        for(int i = 0; i < 26; i++) {
            if(!charOccurances[i]) {
                System.out.println((char)(i+'a'));
                foundSolution = true;
                break;
            }
        }
        if(!foundSolution) {
            String shortest = "";
            for(int i = 0; i < words.length;i++) {
                String temp = "";
                boolean wordSolFound = false;
                for(int j = 0; j < words[i].length() && !wordSolFound ; j++) {
                    char c = 'a';
                    
                    while((!checkSubstrings(temp+c))&&c <= 'z') {
                        c++;
                    }
                    if(c > 'z')
                        temp += words[i].charAt(j);
                    else 
                        temp += c;
                    if(checkSubstrings(temp)) {
                        wordSolFound = true;
                        if(shortest.isEmpty() || temp.length() < shortest.length()) 
                            shortest = temp;
                    }
                }
            }
            System.out.println(shortest);
        }
    }
    
    private static boolean checkSubstrings(CharSequence t) {
        for(int i = 0; i < words.length; i++) {
            if(words[i].contains(t))
                return false;
        }
        return true;
    }
    
    private static void findOccurances() {
        for(int i = 0; i < words.length; i++) {
            for(int j = 0; j < words[i].length(); j++) {
                charOccurances[words[i].charAt(j)-'a'] = true;
            }
        }
    }
    
    
    
}
