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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
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import java.util.Scanner;

public class B {
	static int n ;
	static String ans ;
	public static void main(String [] args){
		Scanner in = new Scanner(System.in);
		n = in.nextInt();
		ans="";
		char [][] c = new char[n][]; 
		String [] str = new String[n];
		for(int i = 0 ; i < n ; i++){
			str[i] = in.next();
			c[i] = str[i].toCharArray();
			
		}
		String f = "";
		String ff= "";
		int i = 0 ;
		int j=0;
		while(true){
			Mosjava(c,f,str);
			if(ans.length()!=0) break;
			f=ff+(char)(i+97);
			i++;
			if(i==26){
				ff=""+(char)(j);
				i=0;
				j++;
				j%=26;
			}
		}
		System.out.println(ans);
	}
	public static void Mosjava(char [][] c , String f,String [] str){
		boolean b [] = new boolean[26]; 
		if(f.length()==0){
		for(int i = 0 ; i < n ; i++){
			for(int j = 0 ; j < c[i].length ; j++){
				b[c[i][j]-97]=true;
			}
		}
		for(int i = 0 ; i < 25 ; i++){
			if(!b[i]){
				ans= (f+(char)(i+97));
				return;
			}
		}
		}
		else{
			for(int i = 0 ; i < n ; i++){
				int k = 0;
				while(true){
					int j = str[i].indexOf(f, k);
					if(j==-1){
						break;
					}
					else{
						b[c[i][j+f.length()-1]-97] = true;
						k=j+1;
					}
				}
			}
			for(int i = 0 ; i < 25 ; i++){
				if(!b[i]){
					ans= (f+(char)(i+97));
					return;
				}
			}
		}
		
		
		
	}
}