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 2
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 2
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 1
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import java.io.*;
import java.lang.*;
import java.util.*;

public class VIG {
    public static void main(String[] args) {
	    Scanner scan = new Scanner(System.in);
	    int n = scan.nextInt();
	    String[] str = new String[n];
	    for(int i=0;i<n;i++)
	    	str[i] = scan.next();
	    
	    int[] count = new int[26];
	    for(int i=0;i<n;i++) {
	    	String s = str[i];
	    	for(int j=0;j<s.length();j++) {
	    		count[s.charAt(j)-'a']++;
	    	}
	    }
	    
	    StringBuilder sb = new StringBuilder();
	    for(int i=0;i<26;i++) {
	    	if(count[i] == 0) {
	    		sb.append((char)(i+97));
	    		break;
	    	}
	    }
	    
	    if(sb.length() > 0) {
	    	System.out.println(sb.toString());
	    	return;
	    }

	    for(int i=0;i<26;i++) {
	    	sb.append((char)(i+97));
	    	boolean flag = true;
	    	for(int j=0;j<26;j++) {
	    		sb.append((char)(j+97));
	    		String st = sb.toString();
	    		for(int x=0;x<str.length;x++) {
	    			if(str[x].equals(st)) {
	    				flag = false;
	    				break;
	    			}
	    		}
	    		if(flag) {
	    			System.out.println(sb.toString());
	    			return;
	    		}
	    		sb.deleteCharAt(1);
	    		flag = true;
	    	}
	    	sb.deleteCharAt(0);
	    }
	    
	    if(sb.length() == 0) {
	    	System.out.println("aaa");
	    	return;
	    }
	    
//	    for(int x = 0;x<26;x++) {
//	    	sb.append((char)(x+97));
//		    boolean flag = true;
//		    for(int i=0;i<26;i++) {
//		    	sb.append((char)(i+97));
//		    	String st = sb.toString();
//		    	for(int j=0;j<str.length;j++) {
//		    		if(str[j].equals(st)) {
//		    			flag = false;
//		    			break;
//		    		}
//		    	}
//		    	if(flag) {
//		    		System.out.println(sb);
//		    		return;
//		    	}
//		    	sb.deleteCharAt(sb.length()-1);
//		    }
//	    }
	    
    }
}












