3
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 1
z
 30
b
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p
n
f
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i
j
k
p
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a
c
w
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d
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 5
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 2
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 5
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abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
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abcdefghijklmnopqrst
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ddfhdfhlasjt
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 1
epkotfpkkrhhmuipmtdk
 10
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 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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khbknqbvdkdd
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avptgpjmenfzfxwckmt
 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.*;
import java.util.*;

import static java.lang.Math.*;

public class Main extends PrintWriter {
    BufferedReader in;
    StringTokenizer stok;
    final Random rand = new Random(31);
    final int inf = (int) 1e9;
    final long linf = (long) 1e18;

    final static String IO = "_std";

    char[] add1(char[] s) {
        int n = s.length;
        int p = -1;
        for (int i = n - 1; i >= 0; i--) {
            if (s[i] != 'z') {
                p = i;
                break;
            }
        }
        if (p != -1) {
            s[p]++;
            return s;
        }
        char[] res = new char[n + 1];
        for (int i = 0; i < res.length; i++) {
            res[i] = 'a';
        }
        return res;
    }

    void solve() throws IOException {
        int n = nextInt();
        Set<String> ss = new HashSet<>();
        for (int i = 0; i < n; i++) {
            String s = next();
            for (int j = 0; j < s.length(); j++) {
                for (int k = j; k < s.length(); k++) {
                    ss.add(s.substring(j, k + 1));
                }
            }
        }
        char[] t = new char[] {'a'};
        while (ss.contains(String.valueOf(t))) {
            t = add1(t);
        }
        println(String.valueOf(t));
    }

    Main() {
        super(System.out);
        in = new BufferedReader(new InputStreamReader(System.in));
    }

    Main(String fileIn, String fileOut) throws IOException {
        super(fileOut);
        in = new BufferedReader(new FileReader(fileIn));
    }

    public static void main(String[] args) throws IOException {
        Main main;
        if ("_std".equals(IO)) {
            main = new Main();
        } else if ("_iotxt".equals(IO)) {
            main = new Main("input.txt", "output.txt");
        } else {
            main = new Main(IO + ".in", IO + ".out");
        }
        main.solve();
        main.close();
    }

    String next() throws IOException {
        while (stok == null || !stok.hasMoreTokens()) {
            String s = in.readLine();
            if (s == null) {
                return null;
            }
            stok = new StringTokenizer(s);
        }
        return stok.nextToken();
    }

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

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

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

    int[] nextIntArray(int len) throws IOException {
        int[] a = new int[len];
        for (int i = 0; i < len; i++) {
            a[i] = nextInt();
        }
        return a;
    }

    int[] nextIntArraySorted(int len) throws IOException {
        int[] a = nextIntArray(len);
        shuffle(a);
        Arrays.sort(a);
        return a;
    }

    void shuffle(int[] a) {
        for (int i = 1; i < a.length; i++) {
            int x = rand.nextInt(i + 1);
            int _ = a[i];
            a[i] = a[x];
            a[x] = _;
        }
    }

    void shuffleAndSort(int[] a) {
        shuffle(a);
        Arrays.sort(a);
    }

    boolean nextPermutation(int[] p) {
        for (int a = p.length - 2; a >= 0; --a) {
            if (p[a] < p[a + 1])
                for (int b = p.length - 1; ; --b)
                    if (p[b] > p[a]) {
                        int t = p[a];
                        p[a] = p[b];
                        p[b] = t;
                        for (++a, b = p.length - 1; a < b; ++a, --b) {
                            t = p[a];
                            p[a] = p[b];
                            p[b] = t;
                        }
                        return true;
                    }
        }
        return false;
    }

    <T> List<T>[] createAdjacencyList(int n) {
        List<T>[] res = new List[n];
        for (int i = 0; i < n; i++) {
            res[i] = new ArrayList<>();
        }
        return res;
    }
}