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
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gecntpcmoojfwp
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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
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

#include <bits/stdc++.h>
void next(char *s) {
  int k = (int)strlen(s), l = 0;
  k--;
  while (k >= 0 && s[k] == 'z') {
    k--;
  }
  if (k < 0) {
    k = (int)strlen(s);
    while (l <= k) {
      s[l] = 'a';
      l++;
    }
    s[l] = '\0';
  } else {
    s[k]++;
  }
}
void swapc(int n, char a[n][23], int *hj, int i, int j) {
  char temp[22];
  strcpy(temp, a[i]);
  strcpy(a[i], a[j]);
  strcpy(a[j], temp);
  int t;
  t = hj[i];
  hj[i] = hj[j];
  hj[j] = t;
}
void qsc(int n, char arr[n][23], int *hj, int ll, int lr) {
  int i, last;
  if (ll >= lr) return;
  swapc(n, arr, hj, ll, (ll + lr) / 2);
  last = ll;
  for (i = ll + 1; i <= lr; i++)
    if (hj[i] < hj[ll] || (hj[i] == hj[ll] && strcmp(arr[i], arr[ll]) < 0))
      swapc(n, arr, hj, ++last, i);
  swapc(n, arr, hj, ll, last);
  qsc(n, arr, hj, ll, last - 1);
  qsc(n, arr, hj, last + 1, lr);
}
int main() {
  int n, i = 0, k = 0;
  scanf("%d", &n);
  char a[n][23], mem[410][23];
  int hj[410] = {0};
  while (i < n) {
    scanf("%s", a[i]);
    int j = (int)strlen(a[i]), st = 0, ft = 0;
    while (st < j) {
      ft = st;
      while (ft < j) {
        strncpy(mem[k], &a[i][st], ft - st + 1);
        mem[k][ft - st + 1] = '\0';
        hj[k] = (int)strlen(mem[k]);
        k++;
        ft++;
      }
      st++;
    }
    i++;
  }
  qsc(410, mem, hj, 0, k - 1);
  i = 1;
  while (i < k) {
    if (mem[i][hj[i] - 1] - mem[i - 1][hj[i - 1] - 1] > 1) break;
    i++;
  }
  next(mem[i - 1]);
  printf("%s", mem[i - 1]);
  return 0;
}
