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.util.HashSet;
import java.util.Scanner;


public class Solution {
    
    static String find(HashSet<String> h, char[] c, int ind) {
        if (ind == c.length) {
            String res = new String(c); 
            if (!h.contains(res)) return res;
            return null;
        }
        
        for (char ch = 'a'; ch <= 'z'; ch++) {
            c[ind] = ch;
            String res = find(h, c, ind + 1);
            if (res != null) return res;
        }
        
        return null;
    }
    
    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        HashSet<String> h = new HashSet<String>();
        int[] sl = new int[5];
        sl[0] = 1;
        for (int i = 1; i < 5; i++) sl[i] = sl[i - 1] * 26;
        
        for (int i = 0; i < n; i++) {
            String s = in.next();
            for (int l = 0; l < s.length(); l++) 
                for (int r = l + 1; r <= s.length(); r++) {
                    String t = s.substring(l, r); 
                    if (r - l <= 3 && !h.contains(t))
                        sl[r - l]--;
                    h.add(t);
                }
        }
        int len = 1;
        while (sl[len] == 0) len++;
        
        String ans = find(h, new char[len], 0);
        System.out.println(ans);
    }
    
}
