3
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 1
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 30
b
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k
p
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a
c
w
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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
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 5
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abcdefghijklmnopqrst
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 1
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 10
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 5
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abcdefghijklmnopqrst
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 2
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 1
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 30
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 30
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 1
b

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

import java.util.*;
public class codeforces2 {
    public static void main(String args[]){
        Scanner sc=new Scanner(System.in);
        int a=sc.nextInt();
        String str[]=new String[a];
        String ans="";
        
        for(int i=0;i<a;i++){
            String b=sc.next ();
            str[i]=b;
        }
        
            
        int flag=0;     
outer:          for(char i='a';i<='z';i++){
    
                    for(String j : str){
                        char[] c=j.toCharArray();
                        for(int k=0;k<c.length;k++){
                            if(c[k]==i){
                                if(i=='z'){
                                    break outer;
                                }
                                else{
                                    continue outer;
                                }
                            }
                        }
                    }
                    flag=1;
                    System.out.println(i);
                    break outer;
                }
            
        Arrays.sort(str);
        if(flag==0){
            ans="a";
            int e=1;
outer:  for(char i='a';i<='z';i++){
            
anuj:       for(char y='a';y<='z';y++){
                ans=ans.substring(0,e)+y;
            //  System.out.println(ans);
                
            for(String j : str){
                int b=ans.compareTo(j);
                if(b==0)continue anuj;
                if(b<0){
                    System.out.print(ans);
                    break outer;
                }
            }
            
        }
        ans=ans.substring(0,e)+i;e++;   
                
        }
        
        
    }
    }
}
