3
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 1
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 30
b
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a
c
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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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 1
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 10
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 5
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 2
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 1
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 30
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skrvnyz
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 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.util.*;
public class Main1{
	public static void main(String []args){
		Scanner s=new Scanner(System.in);
		int n=s.nextInt();
		s.nextLine();
		boolean total[]=new boolean[26];
		String input[]=new String[n];
		int array[][]=new int[n][26];
		for(int i=0;i<n;i++)
			input[i]=s.nextLine();

		for(int i=0;i<26;i++)
			total[i]=false;

		for(int i=0;i<n;i++){
			for(int j=0;j<input[i].length();j++){
				array[i][input[i].charAt(j)-'a']++;
				if(!total[input[i].charAt(j)-'a'])
					total[input[i].charAt(j)-'a']=true;
			}
		}
		String ans="";
		for(int i=0;i<26;i++){
			if(!total[i]){
				ans+=(char)('a'+i);
				break;
			}
		}
		if(ans.length()==1){
			System.out.println(ans);
			return;
		}
		else{
			HashSet<String> set=new HashSet<String>();
			char first='a';
			int len=2;
			for(;first<='z';first++){
				for(int i=0;i<n;i++){
					for(int j=0;j<input[i].length();j++){
						if(input[i].charAt(j)==first){
							String temp="";
							for(int k=j;k<len;k++){
								temp+=input[i].charAt(k);
							}
							set.add(temp);
						}
					}
				}
				String temp=first+"";
				for(char c='a';c<='z';c++){
					temp+=c;
					if(!set.contains(temp)){
						System.out.println(temp);
						return;
					}
					temp=first+"";
				}
				set.clear();
			}

		}
	}
}
