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

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 *
 * @author David Wu
 */
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);
        TaskB solver = new TaskB();
        solver.solve(1, in, out);
        out.close();
    }

    static class TaskB {
        public void solve(int testNumber, Scanner in, PrintWriter out) {
            TaskB.FastScanner sc = new TaskB.FastScanner();
            int init = Integer.parseInt(sc.nextLine());
            ArrayList<String> strings = new ArrayList<>(init);
            ArrayList<Integer> chars = new ArrayList<>(26);
            StringBuffer allStrings = new StringBuffer();
            for (int i = 0; i < 26; i++) {
                chars.add(i + 97);
            }
            int longestL = Integer.MIN_VALUE;

            for (int i = 0; i < init; i++) {
                String s = sc.nextLine();
                strings.add(s);
                allStrings.append(s + " ");
                longestL = Math.max(longestL, s.length());

                for (int k = 0; k < s.length(); k++) {
                    int t = (int) s.charAt(k);
                    if (chars.contains(t)) {
                        chars.remove((Object) t);
                    }
                }
            }
            StringBuffer output = new StringBuffer();

            if (!chars.isEmpty()) {
                int i = chars.get(0);
                char a = (char) i;
                out.println(a);
            } else {
                boolean con = true;
                for (int j = 0; j < longestL; j++) {
                    if (con) {
                        String temp;
                        for (int k = 97; k < 123; k++) {
                            temp = output.toString();
                            temp += (char) k;

                            if (strings.contains(temp)) {
                                if (k == 122) {
                                    output = new StringBuffer(temp);
                                    break;
                                }
                                continue;
                            } else {
                                if (temp.length() == 1) {
                                    output.append(temp);
                                    break;
                                } else if (TaskB.StringUtils.countMatches(allStrings.toString(), temp) >= 1) {
                                    continue;
                                } else {
                                    out.println(temp);
                                    con = false;
                                    break;
                                }
                            }
                        }
                    } else {
                        break; //end program
                    }
                }

            }
        }

        static class StringUtils {
            public static final int INDEX_NOT_FOUND = -1;

            public static boolean isEmpty(final CharSequence cs) {
                return cs == null || cs.length() == 0;
            }

            public static int countMatches(final CharSequence str, final CharSequence sub) {
                if (isEmpty(str) || isEmpty(sub)) {
                    return 0;
                }
                int count = 0;
                int idx = 0;
                while ((idx = indexOf(str, sub, idx)) != INDEX_NOT_FOUND) {
                    count++;
                    idx += sub.length();
                }
                return count;
            }

            static int indexOf(final CharSequence cs, final CharSequence searchChar, final int start) {
                return cs.toString().indexOf(searchChar.toString(), start);
                //        if (cs instanceof String && searchChar instanceof String) {
                //            // TODO: Do we assume searchChar is usually relatively small;
                //            //       If so then calling toString() on it is better than reverting to
                //            //       the green implementation in the else block
                //            return ((String) cs).indexOf((String) searchChar, start);
                //        } else {
                //            // TODO: Implement rather than convert to String
                //            return cs.toString().indexOf(searchChar.toString(), start);
                //        }
            }

        }

        public static class FastScanner {
            BufferedReader br;

            public FastScanner() {
                br = new BufferedReader(new InputStreamReader(System.in));
            }

            String nextLine() {
                String str = "";
                try {
                    str = br.readLine();
                } catch (IOException e) {
                    e.printStackTrace();
                }
                return str;
            }

        }

    }
}

