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import java.util.Arrays;
import java.util.HashSet;
import java.util.Scanner;


public class Main {

	public static void main(String[] args) {
		
		Scanner in=new Scanner(System.in);
		
		int n=in.nextInt();
		String arr[]=new String[n];
		int abc[]=new int[26];
		HashSet<Character> ch= new HashSet<Character>();
		
		for (int i = 0; i < n; i++) {
			arr[i]=in.next();
			for (int j = 0; j < arr[i].length(); j++) {
				ch.add(arr[i].charAt(j));
				abc[arr[i].charAt(j)-'a']++;
			}
		}	
		String res="";
		if(ch.size()==26){
				Arrays.sort(arr);
				String var=arr[0];
				res=solve(var);
		}
		else{
			for (int i = 0; i < abc.length; i++) {
				if(abc[i]==0){
					res=(char)(i+'a')+"";
					break;
				}
			}
		}
	
		System.out.println(res);
	}

	private static String solve(String var) {
		int lleva=1;
		String newString="";
		
		for (int i = var.length()-1; i >=0; i--) {
			int a=(var.charAt(i)+lleva-'a');
			
			if(a>25){
				a=0;
				lleva=1;
			}
			else
				lleva=0;
			newString=(char)(a+'a')+newString;
		}
		if(lleva==1)newString="a"+newString;
		return newString;
	}

	private static void print(Object... o) {
		System.err.println(Arrays.deepToString(o));
	}

	

}
