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

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 a
 e
 a
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 aa
 a
 ab
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 a

import java.io.InputStreamReader;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.Arrays;
import java.util.LinkedList;
import java.util.Scanner;
import java.util.StringTokenizer;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
	public static void main(String[] args) {
		OutputStream outputStream = System.out;
		Scanner in = new Scanner(System.in);
		PrintWriter out = new PrintWriter(outputStream);
		TaskB solver = new TaskB();
		solver.solve(1, in, out);
		out.close();
	}
}

class TaskB {
	static int numdiv;
	static int nump;
	static String[] names;
	static boolean works = false;
	static String cor = "";
	public void solve(int testNumber, Scanner in, PrintWriter out) {
    	int n = Integer.parseInt(in.nextLine());
  	  names = new String[n];
  	  for(int i = 0; i < n; i++){
  		  names[i] = in.nextLine();
  	  }
  	  for(int i = 2; i < 22; i++){
  		  if(works == true){
  			  System.out.println(cor);
  			  break;
  		  }
  		  else{
  			  recurse(i, 1, "");
  		  }
  	  }
    }
    public static void recurse(int longest, int cur, String bef){
   	 if(works == true){return;}
   	  if(cur != 1){
   		  boolean canbe = true;
   		  for(String word: names){
   			  if(word.indexOf(bef) != -1){
   				  canbe = false;
   				  break;
   			  }
   		  }
   		  if(canbe == true){
   			  works = true;
   			  cor = bef;
   			  //System.out.println(bef);
   			  return;
   		  }
   	  }
   	  if(cur == longest){
   		  return;
   	  }
   	  for(int i = (int)'a'; i <=  (int)'z'; i++){
   		 String nbef = bef + (char)i;
   		 //System.out.println(nbef);
   		  recurse(longest, cur + 1, nbef);
   	  }
     }
}
 
