import java.util.HashSet;
import java.util.Scanner;
import java.util.Set;

public class Main {
	public static void main(String[] args) throws Exception {
		Scanner sc = new Scanner(System.in);
		int n = sc.nextInt();
		Set<String> set = new HashSet<>();
		for (int i = 0; i < n; i++) {
			set.add(sc.next());
		}
		sc.close();

		StringBuilder sb = new StringBuilder();
		for (int i = 0; i < n; i++) {
			sb.append('a');
		}
		for (int i = 0; i < n; i++) {
			String s = sb.toString();
			if (!set.contains(s)) {
				System.out.println(s);
				return;
			}
			sb.setCharAt(i, 'b');
		}
		System.out.println(sb.toString());
	}
}