import java.util.*;

public class Main {
    
    public static void main(String[] args) throws Exception {
        
        Scanner sc = new Scanner(System.in);
        
        int n = sc.nextInt();
        List<String> ngWords = new ArrayList<>();
        while(sc.hasNext()){
            ngWords.add(sc.next());
        }

        sc.close();
        
        char a = 'a';
        char b = 'b';
        StringBuilder sb = new StringBuilder();
        for(int i = 0 ; i < n ; i++){
            sb.append(a);
        }
        
        for(int i = 0 ; i < n ; i++){
            String str = sb.toString();
            if(!ngWords.contains(str)){
                System.out.println(str);
                return;
            }
            sb.setCharAt(i , b);
        }
        
        System.out.println(sb.toString());
        
    }
    
}