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import java.util.*;import java.io.*;
public class Main{
	static String test1="5\n"+
	"threehorses\n"+
	"goodsubstrings\n"+
	"secret\n"+
	"primematrix\n"+
	"beautifulyear\n";
	static String test2 = "4\n"+
	"aa\n"+
	"bdefghijklmn\n"+
	"opqrstuvwxyz\n"+
	"c\n";
	
	static String rndTest(int hm_lines, int linelength){
		String ret = ""+hm_lines+"\n";
		for(int i=0;i<hm_lines;i++)ret+=rndString(linelength)+"\n";
		return ret;
	}
	static String rndString(int length){
		Random rnd = new Random();
		String bob = "";
		while(bob.length()<length)bob+=(char)('a'+rnd.nextInt(26));
		return bob;
	}
	public static void main(String[] args){
		Scanner in = new Scanner(System.in);
	//	long start = System.currentTimeMillis();
		while(in.hasNext()){
			int hm_lines = Integer.parseInt(in.nextLine());
			String[] lines = new String[hm_lines];
			for(int i=0;i<hm_lines;i++){
				lines[i]=in.nextLine();
			}
			int cand = 0;
			while(!original(getString(cand),lines))cand++;
			System.out.println(getString(cand));
//			System.out.println(System.currentTimeMillis()-start);
		}//end while in.hasNext();
	}//main
	private static boolean original(String newstring, String[] oldstrings){
	for(String old:oldstrings)if(old.contains(newstring))return false;
	return true;
	}
	private static String getString(int index){
		if(index<26)return ""+(char)('a'+index);
		else return getString(index/26-1)+(char)('a'+(index%26));
	}
}//class