3
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 1
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 30
b
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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
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 l
 a

import java.util.*;
public class NewProblem{
   public static void main(String[] Args){
      Scanner sc = new Scanner(System.in);
      int n = sc.nextInt();
      String[] s = new String[n];
      boolean done = false;
      String ans = "";
      for(int k = 0; k < n; k++){
         s[k] = sc.next();
      }
      for(int k = 0; k<26 && !done;k++){
         ans = ""+(char)(k + 'a');
         done = true;
         for(int ii = 0;ii < n && done;ii++)
            if(s[ii].contains(ans))
               done = false;
         
      }
      for(int k = 0; k<26 && !done;k++){
         for(int j = 0; j < 26 && !done;j++){
            ans = ""+(char)(k + 'a');
            ans = ans + (char)(j + 'a');
            done = true;
            for(int ii = 0;ii < n && done;ii++)
               if(s[ii].contains(ans))
                  done = false;
         }
      }
      for(int k = 0; k<26 && !done;k++){
         for(int j = 0; j < 26 && !done;j++){
            for(int i = 0; i < 26; i++){
               ans = ""+(char)(k + 'a');
               ans = ans + (char)(j + 'a');
               ans = ans + (char)(i + 'a');
               done = true;
               for(int ii = 0;ii < n && done;ii++)
                  if(s[ii].contains(ans))
                     done = false;
            }
         }
      }
      System.out.println(ans);
   }
}
