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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
b

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import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.PrintWriter;
import java.math.BigInteger;
import java.util.*;

public class B implements Runnable {

    String[] f;
    
    private void solve() throws IOException {
        int n = nextInt();
        
        f = new String[n];
        for (int i = 0; i < n; i++)
            f[i] = nextToken();
        
        Queue<String> Q = new LinkedList<String>();
        for (char c = 'a'; c <= 'z'; c++)
            Q.add(c + "");
        
        while (!Q.isEmpty()) {
            String cur = Q.poll();
            
            if (!contain(cur)) {
                pl(cur);
                break;
            }
            
            for (char c = 'a'; c <= 'z'; c++) {
                Q.add(cur + c);
            }
        }
        
        pl("");
    }

    private boolean contain(String s) {
        for (int i = 0; i < f.length; i++)
            if (f[i].contains(s))
                return true;
        return false;
    }

    public static void main(String[] args) {
        new B().run();
    }

    BufferedReader reader;
    StringTokenizer tokenizer;
    PrintWriter writer;

    public void run() {
        try {
            reader = new BufferedReader(new BufferedReader(new InputStreamReader(System.in)));
            writer = new PrintWriter(System.out);
            tokenizer = null;
            solve();
            reader.close();
            writer.close();
        } catch (Exception e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

    int nextInt() throws IOException {
        return Integer.parseInt(nextToken());
    }

    long nextLong() throws IOException {
        return Long.parseLong(nextToken());
    }

    double nextDouble() throws IOException {
        return Double.parseDouble(nextToken());
    }

    BigInteger nextBigInteger() throws IOException {
        return new BigInteger(nextToken());
    }

    String nextToken() throws IOException {
        while (tokenizer == null || !tokenizer.hasMoreTokens()) {
            tokenizer = new StringTokenizer(reader.readLine());
        }
        return tokenizer.nextToken();
    }

    void p(Object... objects) {
        for (int i = 0; i < objects.length; i++) {
            if (i != 0)
                writer.print(' ');
            writer.print(objects[i]);
        }
    }

    void pl(Object... objects) {
        p(objects);
        writer.println();
    }

    int cc;

    void pf() {
        writer.printf("Case #%d: ", ++cc);
    }

}