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
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vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
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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
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znligfxkgxzkcfeu
liryvzmqwhr
xgrxkgiehxztv
etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
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q
cvdu
a
jos
qy
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nayke
ge
s
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qwad
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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>
int str[1200];
int n;
char s[30];
int cnt;
char ans_1, ans_2;
char base = (char)('a' - 1);
int char2int(char lo, char hi = base) { return 27 * (hi - base) + (lo - base); }
void int2char(int val, char &lo, char &hi) {
  lo = val % 27 + base;
  hi = val / 27 + base;
}
int main() {
  scanf("%d", &n);
  str[cnt++] = 0;
  for (int i = (0); i != (n); i++) {
    scanf("%s", s);
    for (int j = (0); j != (30); j++) {
      if (s[j] == '\0')
        break;
      else
        str[cnt++] = char2int(s[j]);
    }
    for (int j = (0); j != (29); j++) {
      if (s[j] == '\0' || s[j + 1] == '\0')
        break;
      else
        str[cnt++] = char2int(s[j], s[j + 1]);
    }
  }
  str[cnt++] = 729;
  std::sort(str, str + cnt);
  for (int i = (0); i != (cnt - 1); i++) {
    if ((str[i + 1] - str[i] >= 3)) {
      int2char(str[i] + 1, ans_1, ans_2);
      if (ans_1 == '`') int2char(str[i] + 2, ans_1, ans_2);
      break;
    } else if ((str[i + 1] - str[i] == 2)) {
      int2char(str[i] + 1, ans_1, ans_2);
      if (ans_1 == '`') int2char(str[i] + 2, ans_1, ans_2);
      continue;
    }
  }
  if (ans_2 != '`') printf("%c", ans_2);
  printf("%c", ans_1);
  return 0;
}
