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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;
import java.util.ArrayList;

public class NewProblem {
  public static void main(String []args)throws IOException
  {
    BufferedReader br=new BufferedReader(new InputStreamReader(System.in));
    PrintWriter pw=new PrintWriter(new OutputStreamWriter(System.out));
    int n=Integer.parseInt(br.readLine());
    ArrayList<String>p=new ArrayList<String>();
    String abc="abcdefghijklmnopqrstuvwxyz";
    for(int i=0;i<abc.length();i++)p.add(""+abc.charAt(i));
    for(int i=0;i<abc.length();i++)
    {
      char c=abc.charAt(i);
      for(int j=0;j<abc.length();j++)
       p.add(""+c+abc.charAt(j));
    }
    String titulos="";
    for(int i=0;i<n;i++) titulos+=" "+br.readLine();
    for(int i=0;i<p.size();i++){
      if(!titulos.contains(p.get(i))){
    	 pw.print(p.get(i));
    	 break;
      }
    }
    
    pw.flush();
  }
}