3
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 1
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 30
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 5
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 2
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 5
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abcdefghijklmnopqrst
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 2
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 5
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abcdefghijklmnopqrst
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 1
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 10
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 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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 30
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 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

import java.io.BufferedReader;
import java.io.Closeable;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.util.ArrayList;
import java.util.List;
import java.util.StringTokenizer;

public class NewProblem implements Closeable {

    private InputReader in = new InputReader(System.in);
    private PrintWriter out = new PrintWriter(System.out);

    public void solve() {
        int n = in.ni();
        String[] data = new String[n];
        for (int i = 0; i < n; i++) {
            data[i] = in.next();
        }
        char[] result = new char[20];
        int idx = 0;
        while (true) {
            boolean fit = false;
            for (char c = 'a'; c <= 'z'; c++) {
                result[idx] = c;
                if (fit(result, idx, data)) {
                    fit = true;
                    break;
                } 
            }
            if (!fit) {
                result[idx++] = 'a';
            } else break;
        }
        for (int i = 0; i <= idx; i++) {
            out.print(result[i]);
        }
    }
    
    private boolean fit(char[] result, int idx, String[] data) {
        StringBuilder sb = new StringBuilder();
        for (int i = 0; i <= idx; i++) {
            sb.append(result[i]);
        }
        String title = sb.toString();
        boolean fit = true;
        for (String s : data) {
            fit &= !s.contains(title);
        }
        return fit;
    }

    @Override
    public void close() throws IOException {
        in.close();
        out.close();
    }

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

        public InputReader(InputStream stream) {
            reader = new BufferedReader(new InputStreamReader(stream), 32768);
            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 ni() {
            return Integer.parseInt(next());
        }

        public long nl() {
            return Long.parseLong(next());
        }

        public void close() throws IOException {
            reader.close();
        }
    }

    public static void main(String[] args) throws IOException {
        try (NewProblem instance = new NewProblem()) {
            instance.solve();
        }
    }
}
