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

public class Main {

	public static void main(String[] args) {

		Scanner in = new Scanner(System.in);

		int n = in.nextInt();
		String values = "";
		String res = "";
		for (int i = 0; i < n; i++) {
			values += in.next() + " ";
		}

		Queue<String> Q = new LinkedList<String>();
		for (int i = 0; i < 26; i++) {
			Q.add((char) (i + 'a') + "");
		}

		while (!Q.isEmpty()) {
			String node = Q.poll();
			if (!values.contains(node)) {
				res = node;
				break;
			}

			for (int i = 0; i < 26; i++) {
				Q.add(node + (char) (i + 'a') + "");
			}
		}
		System.out.println(res);
	}

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

}
