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
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 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
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jrsrvsvbaiumlmkptn
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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
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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>
using namespace std;
int appear[26];
string str[35];
int cmp(const void *a, const void *b) {
  char *c = (char *)a;
  char *d = (char *)b;
  return strcmp(c, d);
}
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> str[i];
    for (int j = 0; j < str[i].length(); j++) {
      appear[str[i][j] - 'a']++;
    }
  }
  bool flag = 0;
  int k;
  for (k = 0; k < 26; k++) {
    if (!appear[k]) {
      flag = 1;
      break;
    }
  }
  if (flag)
    cout << char('a' + k) << endl;
  else {
    sort(str, str + n);
    int len = str[0].length();
    int cnt = 0;
    for (int i = len - 1; i >= 0; i--) {
      if (str[0][i] == 'z') {
        cnt++;
      }
    }
    if (cnt == len) {
      for (int i = 0; i <= len; i++) str[0][i] = 'a';
    } else {
      int j;
      if (cnt == 0) {
        str[0][len - 1]++;
      } else {
        for (int i = len - 1; i >= 0; i--) {
          if (str[0][i] == 'z') {
            j = i;
            break;
          }
        }
        str[0][j]++;
      }
      if (str[0] != str[1])
        cout << str[0] << endl;
      else {
        while (str[0] == str[1]) {
          int len = str[0].length();
          int cnt = 0;
          for (int i = len - 1; i >= 0; i--) {
            if (str[0][i] == 'z') {
              cnt++;
            }
          }
          if (cnt == len) {
            for (int i = 0; i <= len; i++) str[0][i] = 'a';
          } else {
            int j;
            if (cnt == 0) {
              str[0][len - 1]++;
            } else {
              for (int i = len - 1; i >= 0; i--) {
                if (str[0][i] == 'z') {
                  j = i;
                  break;
                }
              }
            }
            str[0][j]++;
          }
        }
        cout << str[0] << endl;
      }
    }
  }
  return 0;
}
