3
rjnflsbpxqivrcdjptj
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 1
z
 30
b
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n
f
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t
i
j
k
p
h
a
c
w
z
z
p
t
d
o
w
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y
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 5
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 2
hk
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 5
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abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
wjehahqgouqvjienq
 5
ojdfhi
abcdefghijklmnopqrst
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ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
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 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
i
ci
aqs
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cvdu
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jos
qy
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nayke
ge
s
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ubn
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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;
const double PI = 2.0 * acos(0.0);
const double EPS = 1e-6;
char dict[55][25];
int n;
bool check(char str[]) {
  int len = strlen(str);
  for (int i = 0; i < n; i++) {
    for (int k = 0; k < strlen(dict[i]) - len + 1; k++)
      if (strncmp(&dict[i][k], str, len) == 0) return false;
  }
  return true;
}
int main() {
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%s", dict[i]);
  char str[25];
  str[0] = 'a';
  int ptr = 1;
  str[ptr] = NULL;
  while (true) {
    int k = ptr - 1;
    for (; k >= 0; k--) {
      str[k]++;
      if (str[k] > 'z')
        str[k] = 'a';
      else
        break;
    }
    if (k < 0) {
      str[ptr++] = 'a';
      str[ptr] = NULL;
    }
    if (check(str)) {
      puts(str);
      break;
    }
  }
  return 0;
}
