3
rjnflsbpxqivrcdjptj
vpojopbwbwbswdu
rydkiwnugwddcgcrng
 1
z
 30
b
u
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n
f
m
t
i
j
k
p
h
a
c
w
z
z
p
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d
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 5
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 2
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 5
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abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
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 5
ojdfhi
abcdefghijklmnopqrst
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ddfhdfhlasjt
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 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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jhbplhpklrgwnaugdf
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lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
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kopsdfgiopjipw
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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kjexeesevrlowxhghq
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doxnxbjwptnimjmsuijx
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xpbtbpftimnn
khbknqbvdkdd
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skrvnyz
avptgpjmenfzfxwckmt
 30
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 1
b

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

import java.io.*;
import java.math.*;
import java.util.*;

public class Main {
 
    static final double eps = 1e-8;
    static int mod = 1000 * 1000 * 1000 + 9;
    static int n;
    
    public static void main(String[] args) throws IOException {
        Scanner sc = new Scanner(System.in);
        pw = new PrintWriter(System.out);
        try {
            HashSet<String> set = new HashSet<String>();
            int n = nextInt();
            for(int st = 0; st < n; st++)
            {
                String s = next();
                for(int i = 0; i < s.length(); i++)
                    for(int j = i + 1; j <= s.length(); j++)
                            set.add(s.substring(i, j));
            }
            char[] ans = new char[1];
            Queue<String> list = new LinkedList<String>();
            for(char c = 'a'; c <= 'z'; c++)
            {
                ans[0] = c;
                String r = new String(ans);
                if(!set.contains(r))
                    exit(r);
                list.add(r);
            }
            for(int st = 0; st < 30; st++)
            {
                int size = list.size();
                for(int i = 0; i < size; i++)
                {
                    String s = list.remove();
                    for(char c = 'a'; c <= 'z'; c++)
                    {
                        String r = s;
                        r += c;
                        if(!set.contains(r))
                            exit(r);
                        list.add(r);
                    }
                }
                
            }
            
            
            
            
            
            
            
            
            
        }
        finally {
            
            pw.close();
        }
    }


     
     
    


    static StreamTokenizer in = new StreamTokenizer(new BufferedReader(new InputStreamReader(System.in)));
    static BufferedReader bf = new BufferedReader(new InputStreamReader(System.in));
    static StringTokenizer st ;
    static PrintWriter pw;
    static int nextInt() throws IOException {
        in.nextToken();
        return (int) in.nval;
    }
    static long nextLong() throws IOException {
        in.nextToken();
        return (long) in.nval;
    }
    static double nextDouble() throws IOException {
        in.nextToken();
        return in.nval;
    }
    static String next() throws IOException {
        in.nextToken();
        return in.sval;
    }
    static void outArray(int[] O) {
        for(int i = 0; i < O.length - 1; i++)
            pw.print(O[i] + " ");
        pw.println(O[O.length - 1]);
    }
    static void exit(Object arg)
    {
        pw.println(arg);
        pw.flush();
        System.exit(0);
    }
}