3
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 1
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 30
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 5
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 2
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 5
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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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 30
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 1
b

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

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

public class Round170B {
	public static void main(String[] args) throws IOException {
		getLine();
		int n = nextInt();
		for(int i=0; i<n; i++){
			String S = rd.readLine();
			for(int x=0; x<S.length(); x++){
				for(int y=x; y<S.length(); y++){
					set.add(S.substring(x, y+1));
				}
			}
		}
		int len = 1;
		for(int i=0; i<26; i++){
			String cur = ""+((char) ('a'+i));
			if(!set.contains(cur)){
				System.out.println(cur); return;
			}
		}
		for(int i=0; i<26; i++){
			for(int j=0; j<26; j++){
				String cur = ""+((char) ('a'+i));
				cur += (char) ('a'+j);
				if(!set.contains(cur)){
					System.out.println(cur); return;
				}
			}
		}
		for(int i=0; i<26; i++){
			for(int j=0; j<26; j++){
				for(char k='a'; k<='z'; k++){
					String cur = ""+((char) ('a'+i));
					cur += (char) ('a'+j)+k;
					if(!set.contains(cur)){
						System.out.println(cur); return;
					}
				}
			}
		}
		for(int i=0; i<26; i++){
			for(int j=0; j<26; j++){
				for(char k='a'; k<='z'; k++){
					for(char t='a'; t<='z'; t++){
						String cur = ""+((char) ('a'+i));
						cur += (char) ('a'+j)+k+t;
						if(!set.contains(cur)){
							System.out.println(cur); return;
						}
					}
				}
			}
		}
		for(int i=0; i<26; i++){
			for(int j=0; j<26; j++){
				for(char k='a'; k<='z'; k++){
					for(char t='a'; t<='z'; t++){
						for(char s='a'; s<='z'; s++){
							String cur = ""+((char) ('a'+i));
							cur += (char) ('a'+j)+k+t+s;
							if(!set.contains(cur)){
								System.out.println(cur); return;
							}
						}
					}
				}
			}
		}
	}
	
	static HashSet<String> set = new HashSet<String>();
	
	static void tryForLen(int len){
		
	}
	
	
	static void getLine() throws IOException {
		st = new StringTokenizer(rd.readLine());
	}
	
	static int nextInt(){
		return Integer.parseInt(st.nextToken());
	}
	
	static BufferedReader rd = new BufferedReader(new InputStreamReader(System.in));
	static PrintWriter pw = new PrintWriter(System.out);
	static StringTokenizer st;
	
}
