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
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 5
splt
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 2
hk
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 5
zzxpfk
abcdefghijklmnopqrst
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vgnsdfdgfh
 2
rxscdzkkezud
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 5
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abcdefghijklmnopqrst
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qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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kuzoxmiixxbl
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jhbplhpklrgwnaugdf
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lxyzgcmbzulcst
 5
dfijdfhi
abcdefghijklmnopqrst
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kopsdfgiopjipw
jty
 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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qiqehzmgsjdoqd
kjexeesevrlowxhghq
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znligfxkgxzkcfeu
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etrjxdczppafly
jrdgajschhwsci
doxnxbjwptnimjmsuijx
zciwkbvrhgsjhrr
ehh
bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
x
itaubd
xcn
v
x
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ci
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jos
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ge
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t
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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 n;
char c[30];
bool d[30][30];
int main() {
  scanf("%d", &n);
  for (int h = 0; h < n; h++) {
    scanf("%s", c);
    int k = strlen(c);
    for (int j = 0; j < k; j++) {
      d[c[j] - 97][0] = 1;
      if (j != k - 1) d[c[j] - 97][c[j + 1] - 96] = 1;
    }
  }
  for (int h = 0; h < 26; h++)
    if (d[h][0] == 0) {
      printf("%c", char(h + 97));
      return 0;
    }
  for (int h = 0; h < 26; h++)
    for (int j = 1; j < 27; j++)
      if (d[h][j] == 0) {
        printf("%c%c", char(h + 97), char(j + 96));
        return 0;
      }
}
