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
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o
a
u
 5
splt
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 2
hk
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 5
zzxpfk
abcdefghijklmnopqrst
uvwxz
srgkjaskldfkln
vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
uvwxyz
ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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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
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jrsrvsvbaiumlmkptn
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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
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nayke
ge
s
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qwad
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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.BufferedReader;
import java.io.InputStreamReader;
import java.util.LinkedList;
import java.util.Queue;

/**
 * 
 */

/**
 * @author Administrator
 * 
 */
public class A {

	private String[] list;

	public static void main(String[] args) throws Exception {
		A main = new A();
		main.run();
		System.exit(0);
	}

	private void run() throws Exception {
		BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		int N = Integer.parseInt(in.readLine());
		list = new String[N];
		for (int i = 0; i < N; i++)
			list[i] = in.readLine();
		Queue<String> queue = new LinkedList<String>();
		queue.add("");
		while (!queue.isEmpty()) {
			String tmp = queue.poll();
			if (tmp.length() != 0 && check(tmp)) {
				System.out.println(tmp);
				return;
			}
			for (char a = 'a'; a <= 'z'; a++) {
				queue.add(tmp + a);
			}
		}
	}

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