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import java.util.*;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.IOException;
import java.io.PrintWriter;

public class Main {
    public static void main(String[] args) {
        Main app = null;

        try {
            app = new Main();

            int n = app.nextInt();
            Set<String> set = new HashSet<String>();

            for (int i = 0; i < n; ++i) {
                StringBuilder in = new StringBuilder(app.nextToken());

                for (int j = 0; j < in.length() - 1; ++j) {
                    set.add(in.substring(j, j + 2));
                    set.add(in.substring(j, j + 1));
                }

                set.add(in.substring(in.length() - 1, in.length()));
            }

            final int letters = 26;
            String[] answers = new String[letters + letters * letters];

            for (int i = 0; i < letters; ++i) {
                answers[i * (letters + 1)] = String.valueOf((char) ('a' + i));
                for (int j = 0; j < letters; ++j) {
                    answers[i * (letters + 1) + j + 1] = String.valueOf((char) ('a' + i)) + String.valueOf((char) ('a' + j));
                }
            }

            Arrays.sort(answers, new Comparator<String>() {
                @Override
                public int compare(String s, String s1) {
                    if (s.length() != s1.length())
                        return s.length() < s1.length() ? -1 : 1;

                    return s.compareTo(s1);
                }
            });

            for (int i = 0; i < answers.length; ++i) {
                if (!set.contains(answers[i])) {
                    app.writer.println(answers[i]);
                    break;
                }
            }
        } catch(IOException ex) {
            ex.printStackTrace();
        } finally {
            app.writer.close();
            try {
                app.reader.close();
            } catch (IOException ex) {
                ex.printStackTrace();
            }
        }
    }

    private String nextToken() throws IOException {
        if (st == null || !st.hasMoreTokens())
            st = new StringTokenizer(reader.readLine());

        return st.nextToken();
    }

    private int nextInt() throws IOException {
        return Integer.parseInt(nextToken());
    }

    StringTokenizer st;
    PrintWriter writer = new PrintWriter(System.out);
    BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
}