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import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.List;
import java.util.Scanner;
import java.util.ArrayList;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author mthai
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        Scanner in = new Scanner(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        CF_278B solver = new CF_278B();
        solver.solve(1, in, out);
        out.close();
    }

    static class CF_278B {
        public void solve(int testNumber, Scanner input, PrintWriter out) {
            ShortScanner in = new ShortScanner(input);
            int n = in.i();
            List<String> list = new ArrayList<String>();
            for (int i = 0; i < n; ++i) list.add(in.s());

            for (int i = 1; i <= 30; ++i) {
                String ans = cal("", list, 0, i);
                if (ans != null) {
                    out.println(ans);
                    return;
                }
            }
        }

        String cal(String s, List<String> list, int l, int len) {
            if (l == len) return null;
            for (int i = 0; i < 26; ++i) {
                String x = s + (char) ('a' + i);
                boolean ok = true;
                for (String ss : list) {
                    if (ss.contains(x)) {
                        ok = false;
                        break;
                    }
                }
                if (ok) {
                    return x;
                }
            }
            for (int i = 0; i < 26; ++i) {
                String rs = cal(s + (char) ('a' + i), list, l + 1, len);
                if (rs != null) return rs;
            }
            return null;
        }

        class ShortScanner {
            Scanner in;

            ShortScanner(Scanner in) {
                this.in = in;
            }

            int i() {
                return in.nextInt();
            }

            String s() {
                return in.next();
            }

        }

    }
}
