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 1
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 30
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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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 2
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 1
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import java.util.HashSet;
import java.util.Iterator;
import java.util.Scanner;


public class New_Problem{
public static void main(String[] args)
{
    Scanner s=new Scanner(System.in);
    int n=s.nextInt();
    String[] set=new String[n];
    for(int i=0;i<n;i++)
      set[i]=s.next();
    
    
   int j;
   boolean flag=true;
   int count=0;
   String ans="";
   while(true){
      
       flag=true;
       
       for(int i=97;i<=122;i++)
   {
          if(ans.length()==0||(ans.length()==1&&flag==true))
              ans=Character.toString((char)i);
              else
         ans=ans.substring(0,count)+Character.toString((char)i);
        
        for(j=0;j<n;j++){
        
        if(set[j].contains(ans))
        {   
            
            
            break;
            
        }
        }
        if(j==n)
        {
            System.out.println(ans);
            System.exit(0);
        }
        else if(i==122)
        {
            flag=false;
            i=96;
            int l=ans.length();
            ans="";
            
            for(int k=0;k<l;k++)
            ans=ans+Character.toString((char)(count+97));
            count++;
            
        }
        
   }
     
   }
        
        
   
}
}
