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
 e
 b
 aa
 l
 a

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


public class Main {


   public static void main( String[] args ) throws IOException{
      
      BufferedReader in = new BufferedReader( new InputStreamReader( System.in ) );
      int n = Integer.parseInt( in.readLine() );
      StringBuilder sb = new StringBuilder(), tmp;
      TreeSet< String > subs = new TreeSet<>();
      for( int i = 0; i < n; i++ ){
         String word = in.readLine();
         for( int j = 0; j < word.length(); j++ ){
            for( int k = 0; k < word.length() + 1; k++ ){
               if( j < k )
                  subs.add( word.substring( j, k ) );
            }
         }
      }
      String title = "";
      ret:
      while( true ){
         for( char i = 'a'; i <= 'z'; i++ ){
            tmp = new StringBuilder( sb.toString() + i );
            if( !subs.contains( tmp.toString() ) ){
               title = tmp.toString();
               break ret;
            }
         }
         sb.append( 'a' );
      }

      p( title );
           
   }
   public static void pln( String s ){
      System.out.println( s );
   }
   public static void p( String s ){
      System.out.print( s );
   }
}
