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 30
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 2
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 5
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 2
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 5
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 1
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 10
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 2
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 1
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 1
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import java.io.*;
import java.util.*;

public class Main {

    private static void solve(InputReader in, OutputWriter out) {
        int n = in.nextInt();

        Set<String> dict = new HashSet<>();
        for (int i = 0; i < n; i++) {
            String s = in.next();

            for (int j = 0; j < s.length(); j++) {
                String st = String.valueOf(s.charAt(j));
                dict.add(st);
                if (j > 0) dict.add(s.charAt(j - 1) + st);
            }
        }

        boolean isFound = false;
        for (int i = 'a'; i <= 'z'; i++) {
            String st = String.valueOf((char) i);

            if (!dict.contains(st)) {
                out.print(st);
                isFound = true;
                break;
            }
        }
        
        if (!isFound) {
            for (int i = 'a'; i <= 'z'; i++) {
                for (int j = 'a'; j <= 'z'; j++) {
                    String st = String.valueOf((char) i) + (char) j;

                    if (!dict.contains(st)) {
                        out.print(st);
                        isFound = true;
                        break;
                    }
                }

                if (isFound) break;
            }
        }

        if (!isFound) out.print("NOT FOUND!");
    }

    private static void shuffleArray(int[] array) {
        int index;
        Random random = new Random();
        for (int i = array.length - 1; i > 0; i--) {
            index = random.nextInt(i + 1);
            if (index != i) {
                array[index] ^= array[i];
                array[i] ^= array[index];
                array[index] ^= array[i];
            }
        }
    }

    public static void main(String[] args) {
        InputReader in = new InputReader(System.in);
        OutputWriter out = new OutputWriter(System.out);
        solve(in, out);
        in.close();
        out.close();
    }

    private static class InputReader {
        private BufferedReader br;
        private StringTokenizer st;

        InputReader(InputStream is) {
            br = new BufferedReader(new InputStreamReader(is));
            st = null;
        }

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

        String next() {
            while (st == null || !st.hasMoreTokens()) {
                String line = nextLine();
                if (line == null) return null;
                st = new StringTokenizer(line);
            }
            return st.nextToken();
        }

        byte nextByte() {
            return Byte.parseByte(next());
        }

        short nextShort() {
            return Short.parseShort(next());
        }

        int nextInt() {
            return Integer.parseInt(next());
        }

        long nextLong() {
            return Long.parseLong(next());
        }

        double nextDouble() {
            return Double.parseDouble(next());
        }

        void close() {
            try {
                br.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }

    private static class OutputWriter {
        BufferedWriter bw;

        OutputWriter(OutputStream os) {
            bw = new BufferedWriter(new OutputStreamWriter(os));
        }

        void print(int i) {
            print(Integer.toString(i));
        }

        void println(int i) {
            println(Integer.toString(i));
        }

        void print(long l) {
            print(Long.toString(l));
        }

        void println(long l) {
            println(Long.toString(l));
        }

        void print(double d) {
            print(Double.toString(d));
        }

        void println(double d) {
            println(Double.toString(d));
        }

        void print(boolean b) {
            print(Boolean.toString(b));
        }

        void println(boolean b) {
            println(Boolean.toString(b));
        }

        void print(char c) {
            try {
                bw.write(c);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        void println(char c) {
            println(Character.toString(c));
        }

        void print(String s) {
            try {
                bw.write(s);
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        void println(String s) {
            print(s);
            print('\n');
        }

        void close() {
            try {
                bw.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}
