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 1
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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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 aa
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import java.io.*;
import java.lang.reflect.Array;
import java.math.BigInteger;
import java.util.*;

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



final class Solution implements Runnable {

    int solve() throws Throwable {
        int n = in.nextInt();
        TreeSet<String> subStrings = new TreeSet<>();
        for (int i = 0; i < n; i++) {
            String s = in.nextToken();
            for (int len = 0; len < Math.min(4, s.length()); len++)
                for (int j = 0; j + len < s.length(); j++)
                    subStrings.add(s.substring(j, j + len + 1));
        }
        StringBuffer sb = new StringBuffer();
        TreeSet<String>[] answers = new TreeSet[3];
        for (int i = 0; i < answers.length; i++)
            answers[i] = new TreeSet<>();
        for (int i = 'a'; i <= 'z'; i++)
            for (int j = 'a' - 1; j <= 'z'; j++)
                for (int k = 'a' - 1; k <= 'z'; k++) {
                    sb.delete(0, sb.length());
                    sb.append((char)i);
                    if (j != 'a' - 1) {
                        sb.append((char)j);
                        if (k != 'a' - 1)
                            sb.append((char)k);
                    }
                    if (!subStrings.contains(sb.toString())) {
                        answers[sb.length() - 1].add(sb.toString());
                    }
                }
        if (answers[0].size() != 0)
            out.println(answers[0].pollFirst());
        else if (answers[1].size() != 0)
            out.println(answers[1].pollFirst());
        else
            out.println(answers[2].pollFirst());


        return 0;
    }


    FastScanner in;
    PrintWriter out;

    @Override
    public void run() {
        try {
            boolean ONLINE_JUDGE = false;
            try {
                Locale.setDefault(Locale.US);
                ONLINE_JUDGE = System.getProperty("ONLINE_JUDGE") != null;
            } catch (SecurityException e) {
                System.err.println("Operation forbidden");
            }


            long time = System.currentTimeMillis();
            if (ONLINE_JUDGE) {
                in = new FastScanner(new InputStreamReader(System.in));
                out = new PrintWriter(System.out);
            } else {
                in = new FastScanner(new FileReader("input.txt"));
                out = new PrintWriter("output.txt");
            }

            int ret = solve();
            out.close();
            in.close();

            long delta = System.currentTimeMillis() - time;

            long freeMem = Runtime.getRuntime().freeMemory();
            long totalMem = Runtime.getRuntime().totalMemory();

            System.err.printf("Time: %.3f s\n", delta / 1000.0f);
            System.err.printf("Memory: %.3f MB\n", (float)(totalMem - freeMem) / (1 << 20));

            if (ret != 0)
                System.exit(ret);

        } catch (Throwable t) {
            t.printStackTrace(System.err);
            System.exit(-1);
        }
    }
}

@SuppressWarnings("unused")
final class FastScanner {
    BufferedReader bufferedReader;
    StringTokenizer stringTokenizer;

    public FastScanner(Reader r) {
        bufferedReader = new BufferedReader(r);
    }

    public String nextToken() throws IOException, NullPointerException {
        while (stringTokenizer == null || !stringTokenizer.hasMoreTokens())
            stringTokenizer = new StringTokenizer(bufferedReader.readLine());
        return stringTokenizer.nextToken();
    }

    public String nextLine() throws IOException {
        return bufferedReader.readLine();
    }

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

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

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

    public float nextFloat() throws IOException {
        return Float.parseFloat(nextToken());
    }

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

    public int read() throws IOException {
        return bufferedReader.read();
    }

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

    public int[] readIntArray(int n) throws Throwable {
        int[] array = new int[n];
        for (int i = 0; i < n; i++)
            array[i] = nextInt();
        return array;
    }

    @Override
    protected void finalize() throws Throwable {
        super.finalize();
        close();
    }
}

final class DisjointSetUnion {
    int[] parent;
    int[] rank;

    public DisjointSetUnion(int size) {
        parent = new int[size];
        rank = new int[size];
        for (int i = 0; i < size; i++)
            parent[i] = i;
    }

    public int getLeader(int element) {
        if (parent[element] == element)
            return element;
        else
            return parent[element] = getLeader(parent[element]);
    }

    public boolean union(int x, int y) {
        x = getLeader(x);
        y = getLeader(y);
        if (x == y)
            return true;
        if (rank[x] < rank[y]) {
            int temp = x;
            x = y;
            y = temp;
        }
        parent[y] = x;
        if (rank[x] == rank[y])
            ++rank[x];
        return false;
    }
}

@SuppressWarnings("unused")
final class Utils {
    private Utils() {}

    public static long sqr(long x) {
        return x * x;
    }

    public static int gcd(int x, int y) {
        while (y > 0) {
            x %= y;
            int temp = x;
            x = y;
            y = temp;
        }
        return x;
    }

    public static int lcm(int x, int y) {
        return x / gcd(x, y) * y;
    }

    public static long gcd(long x, long y) {
        while (y > 0) {
            x %= y;
            long temp = x;
            x = y;
            y = temp;
        }
        return x;
    }

    public static long lcm(long x, long y) {
        return x / gcd(x, y) * y;
    }

    public static long modInverse(long x, long mod) {
        return BigInteger.valueOf(x).modInverse(BigInteger.valueOf(mod)).longValue();
    }

    static Random r = new Random(System.nanoTime());

    public static<T> void shuffle(T[] array) {
        int n = array.length;
        for (int i = 0; i < n; i++) {
            int j = r.nextInt(n);
            T t = array[i];
            array[i] = array[j];
            array[j] = t;
        }
    }

    public static void shuffle(int[] array) {
        int n = array.length;
        for (int i = 0; i < n; i++) {
            int j = r.nextInt(n);
            int t = array[i];
            array[i] = array[j];
            array[j] = t;
        }
    }
}