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 1
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import java.util.*;
import java.io.*;

public class Main {

	BufferedReader in;
	StringTokenizer str = null;
	
	private String next() throws Exception{
		if (str == null || !str.hasMoreElements())
			str = new StringTokenizer(in.readLine());
		return str.nextToken();
	}
	
	private int nextInt() throws Exception{
		return Integer.parseInt(next());
	}
	
	private long nextLong() throws Exception{
		return Long.parseLong(next());
	}
	
	private double nextDouble() throws Exception{
		return Double.parseDouble(next());
	}
	int n;
	String a[];
	public void run() throws Exception{
		in =  new BufferedReader(new InputStreamReader(System.in));
		n = nextInt();
		a = new String[n];
		for(int i=0;i<n;i++) a[i] = next();
		String s = "a";
		int c = 0;
		while(true){
			if(check(s)){
				System.out.println(s);
				return;
			}
			s = getNext(s);
		}
	}

	private String getNext(String s){
		int l = s.length();
		char c = s.charAt(l-1);
		if (c < 'z'){
			return s.substring(0, l-1) + (char)(c+1);
		}
		for(int i=0;i<l;i++){
			if (s.charAt(i) !='z'){
				String tail = "";
				for(int j=i+1;j<l;j++)
					tail+="a";
				return s.substring(0, i) + (char)(s.charAt(i)+1) + tail;
			}
		}
		String t = "";
		for(int i=0;i<l+1;i++)
			t+="a";
		return t;
	}
	
	private boolean check(String s){
		for(int i=0;i<n;i++){
			if (a[i].indexOf(s) != -1)
				return false;
		}
		return true;
	}
		
	public static void main(String[] args) throws Exception{
		new Main().run();
	}
}
