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
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qiqehzmgsjdoqd
kjexeesevrlowxhghq
judikhzkj
z
xo
lsdzypkfqro
sshgcxsky
gecntpcmoojfwp
svmytmcfhc
jrsrvsvbaiumlmkptn
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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
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qwad
gvz
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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.*;
import java.util.HashSet;
import java.util.Set;
import java.util.StringTokenizer;

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

    private void solve() throws IOException {

        StringBuilder c = new StringBuilder("a");
        Set<String> xs = new HashSet<String>();

        int n = nextInt();

        while (n-- > 0) {
            xs.add(next());
        }

        boolean contains = true;
        while (contains) {
            contains = false;
            for (String x : xs) {
                if (x.contains(c)) {
                    while (x.contains(c)) {
                        char lastChar = c.charAt(c.length() - 1);
                        if (lastChar >= 'z') {
                            if (c.length() > 1) {
                                c.setCharAt(0, (char) (c.charAt(0) + 1));
                                c.setCharAt(1, 'a');
                            } else {
                                c = new StringBuilder(c.toString().replaceAll(".", "a")).append('a');
                            }
                        } else {
                            c.setCharAt(c.length() - 1, (char) (lastChar + 1));
                        }
                    }
                    contains = true;
                }
            }
        }

        out.print(c.toString());
    }


    private PrintWriter out;
    private StringTokenizer st;
    private BufferedReader in;

    void run() {
        try {
            in = new BufferedReader(new InputStreamReader(System.in));
            out = new PrintWriter(new OutputStreamWriter(System.out));
            solve();
            in.close();
            out.close();
        } catch (Throwable e) {
            e.printStackTrace();
            System.exit(1);
        }
    }

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

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