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import java.io.PrintWriter;
import java.util.Scanner;

/**
 * <a href="http://codeforces.ru/problemset/problem/278/B"/>
 * 
 * @author pvasilyev
 * @since 22 Jan 2014
 */
public class Problem099 {

    public static void main(String[] args) {
        final Scanner reader = new Scanner(System.in);
        final PrintWriter writer = new PrintWriter(System.out);

        solve(reader, writer);

        reader.close();
        writer.close();
    }

    private static void solve(final Scanner reader, final PrintWriter writer) {
        final int n = reader.nextInt();
        String[] strings = new String[n];
        for (int i = 0; i < strings.length; i++) {
            strings[i] = reader.next();
        }
        int i = 0;
        String currentString;
        while (true) {
            currentString = convertToString(i++);
            if (notASuffixInSomeString(currentString, strings)) {
                break;
            }
        }

        writer.println(currentString);
    }

    private static String convertToString(int number) {
        final StringBuilder stringBuilder = new StringBuilder();
        do {
            stringBuilder.append((char)('a' + (number % 26)));
            number = number / 26 - 1;
        } while (number >= 0);
        return stringBuilder.reverse().toString();
    }

    private static boolean notASuffixInSomeString(final String currentString, final String[] strings) {
        for (int i = 0; i < strings.length; i++) {
            String string = strings[i];
            if (string.contains(currentString)) {
                return false;
            }
        }

        return true;
    }

}
