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import java.util.*;
import java.util.LinkedList;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.util.StringTokenizer;
import java.math.BigInteger;

public class tmp {
    public static void main(String [] args) throws Exception{
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
  //      StringTokenizer st = new StringTokenizer(in.readLine());
        StringBuilder sb = new StringBuilder();
        
        int n = Integer.parseInt(in.readLine());
        
        for(int i=0; i<n; i++)
            sb.append(in.readLine()).append(":");
        
        String s="";
        for(int i=0; i<26; i++){
            s = (char)(i+'a')+"";
            if(sb.indexOf(s) < 0){
                System.out.println(s);
                return;
            }
        }
        
        for(int i = 0; i<26; i++){
            s = (char)(i+'a')+"";
            for(int j=0; j<26; j++){
                s += (char)(j+'a')+"";
                if(sb.indexOf(s) < 0){
                    System.out.println(s);
                    return;
                }
                s = s.substring(0,1);
            }
        }
        
        
        
        
        
        
        
        
        
        
        
        
        
    }
    
    public static long sum(int[][] arr, int start, int end){
        long out = 0;
        for(int i=start; i<=end; i++){
            out+=arr[i][end];
        }
        return out;
    }
    
    public static int isPalindrome(String s){
        for(int i=0; i<=s.length()/2; i++){
            if(s.charAt(i) != s.charAt(s.length()-1-i))
                return 0;
        }
        return 1;
    }
    
    
    
    
    
    
    
    


    
    
    
    
}

    
