3
rjnflsbpxqivrcdjptj
vpojopbwbwbswdu
rydkiwnugwddcgcrng
 1
z
 30
b
u
p
n
f
m
t
i
j
k
p
h
a
c
w
z
z
p
t
d
o
w
u
q
l
y
i
o
a
u
 5
splt
ohqykk
xqpz
knojbur
pmfm
 2
hk
obsp
 5
zzxpfk
abcdefghijklmnopqrst
uvwxz
srgkjaskldfkln
vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
gtpr
kuzoxmiixxbl
crgqtuo
jhbplhpklrgwnaugdf
zuxdaat
aycv
qwghrkqwkobrgevsjrk
tdxgc
lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
o
qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
lsdzypkfqro
sshgcxsky
gecntpcmoojfwp
svmytmcfhc
jrsrvsvbaiumlmkptn
s
wpcsovfjlyspviflk
ktvyzvddgllht
szahigtmklglrcocbo
znligfxkgxzkcfeu
liryvzmqwhr
xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
zbi
q
cvdu
a
jos
qy
fjf
nayke
ge
s
q
qwad
gvz
o
ubn
nzoc
h
wnjwjrf
t
bq
bw
 1
b

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

import java.io.OutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.PrintWriter;
import java.util.TreeMap;
import java.util.StringTokenizer;
import java.io.IOException;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.io.InputStream;

/**
 * Built using CHelper plug-in
 * Actual solution is at the top
 */
public class Main {
    public static void main(String[] args) {
        InputStream inputStream = System.in;
        OutputStream outputStream = System.out;
        InputReader in = new InputReader(inputStream);
        PrintWriter out = new PrintWriter(outputStream);
        B solver = new B();
        solver.solve(1, in, out);
        out.close();
    }

    static class B {
        public void solve(int testNumber, InputReader in, PrintWriter out) {
            int n = in.nextInt();

            B.Trie trie = new B.Trie();
            for (int i = 0; i < n; i++) {
                String s = in.next();
                for (int j = 0; j < s.length(); j++) {
                    for (int k = j + 1; k <= s.length(); k++) {
                        trie.add(s.substring(j, k));
                    }
                }
            }

            for (int l = 1; ; l++) {
                String s = trie.smallest(l);
                if (s != null) {
                    out.println(s);
                    break;
                }
            }
        }

        static class Trie {
            TreeMap<Character, B.Trie> children = null;
            boolean end = false;

            B.Trie next(char ch) {
                if (children == null) {
                    children = new TreeMap<>();
                }

                if (children.containsKey(ch)) {
                    return children.get(ch);
                } else {
                    B.Trie child = new B.Trie();
                    children.put(ch, child);
                    return child;
                }
            }

            void add(String s) {
                B.Trie current = this;
                for (char c : s.toCharArray()) {
                    current = current.next(c);
                }
                current.end = true;
            }

            String smallest(int n) {

                if (children == null) {
                    String s = "";
                    for (int k = 0; k < n; k++) {
                        s += "a";
                    }
                    return s;
                }

                if (n == 1) {
                    for (char c = 'a'; c <= 'z'; c++) {
                        if (!children.containsKey(c)) {
                            return String.format("%c", c);
                        }
                    }
                    return null;
                }

                for (char c = 'a'; c <= 'z'; c++) {
                    if (children.containsKey(c)) {
                        String p = next(c).smallest(n - 1);
                        if (p != null) {
                            return String.format("%c%s", c, p);
                        }
                    }
                }

                return null;
            }

        }

    }

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

    }
}

