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 2
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 5
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 1
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 10
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 1
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 1
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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.Comparator;
import java.util.HashSet;
import java.util.List;
import java.util.Set;
import java.util.StringTokenizer;
import java.util.TreeSet;

import static java.lang.Math.abs;
import static java.lang.Math.max;
import static java.lang.Math.min;
import static java.lang.Math.rint;

/**
 * Author: Maxim Novik
 */
public class Solution {


    public static void main(String[] args) {
        InputStream inputStream = System.in;
        InputReader in = new InputReader(inputStream);
        int stringsNumber = in.nextInt();
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i < stringsNumber; ++i) {
            sb.append(in.next()).append("*");
        }
        String text = sb.toString();
        char letter = 'a';
        for (int i = 0; i < 26; ++i) {
            if (!text.contains(Character.toString(letter))) {
                System.out.println(letter);
                return;
            }
            ++letter;
        }
        char[] pair = {'a', 'a'};
        while (pair[0] < 'z') {
            while (pair[1] < 'z') {
                String s = String.valueOf(pair);
                if (!text.contains(s)) {
                    System.out.println(s);
                    return;
                }
                ++pair[1];
            }
            ++pair[0];
            pair[1] = 'a';
        }
    }


    private static Set<String> getUniqueSubstrings(String s) {
        Set<String> result = new TreeSet<String>(new Comparator<String>() {
            @Override
            public int compare(String s1, String s2) {
                if (s1.length() == s2.length()) {
                    return s1.compareTo(s2);
                }
                return s1.length() - s2.length();
            }
        });
        for (int i = 0; i < s.length(); ++i) {
            for (int j = 1; j < s.length() - i; ++j) {
                result.add(s.substring(i, i + j));
            }
        }
        return result;
    }


    static class InputReader {
        private BufferedReader reader;
        private StringTokenizer tokenizer;

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream));
            tokenizer = null;
        }

        public String next() {
            while (tokenizer == null || !tokenizer.hasMoreTokens()) {
                try {
                    tokenizer = new StringTokenizer(reader.readLine());
                } catch (IOException e) {
                    throw new RuntimeException(e);
                }
            }
            return tokenizer.nextToken();
        }

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

    }
}


