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
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 5
splt
ohqykk
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pmfm
 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
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jhbplhpklrgwnaugdf
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lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
uvwxy
kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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kjexeesevrlowxhghq
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doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
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cvdu
a
jos
qy
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nayke
ge
s
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ubn
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h
wnjwjrf
t
bq
bw
 1
b

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

#include <bits/stdc++.h>
using namespace std;
char s[50][50];
int main() {
  int i, j, k[50], n;
  char *ptr, str[50];
  cin >> n;
  for (i = 1; i <= n; i++) {
    cin >> s[i];
    k[i] = strlen(s[i]);
  }
  str[0] = 'a';
  str[1] = '\0';
  int g = 0;
  while (1) {
    int f = 0;
    for (i = 1; i <= n; i++) {
      ptr = strstr(s[i], str);
      if (ptr) {
        f = 1;
      }
    }
    if (!f) break;
    if (str[g] == 'z') {
      int ff = 0;
      for (i = g - 1; i >= 0; i--)
        if (str[i] != 'z') {
          str[i] += 1;
          ff = 1;
          break;
        }
      str[g] = 'a';
      if (!ff) {
        g++;
        str[g] = 'a';
        str[g + 1] = '\0';
      }
    } else {
      str[g] = str[g] + 1;
    }
  }
  puts(str);
  return 0;
}
