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
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 5
splt
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 2
hk
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 5
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abcdefghijklmnopqrst
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 2
rxscdzkkezud
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 5
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abcdefghijklmnopqrst
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ddfhdfhlasjt
qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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jhbplhpklrgwnaugdf
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 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
waiphwcqrrinr
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doxnxbjwptnimjmsuijx
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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itaubd
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 1
b

a
 a
 e
 a
 a
 y
 b
 aa
 a
 ab
 z
 e
 b
 aa
 l
 a

#include <bits/stdc++.h>
int abs(int a) {
  if (a < 0) return -1 * a;
  return a;
}
int big(int a, int b) {
  if (a > b) return a;
  return b;
}
int small(int a, int b) {
  if (a < b) return a;
  return b;
}
int compare(const void *a, const void *b) {
  int *p1 = (int *)a;
  int *p2 = (int *)b;
  return *p1 - *p2;
}
char st[22];
char s[30][22];
int n, found = 0;
void f(int l, int step) {
  int i;
  if (found) return;
  if (step == l) {
    st[step] = '\0';
    for (i = 0; i < n; i++)
      if (strstr(s[i], st) != NULL) break;
    if (i == n) found = 1;
    return;
  }
  for (i = 0; i < 26; i++) {
    if (found) return;
    st[step] = i + 'a';
    f(l, step + 1);
  }
}
int main() {
  int i, j, k, x, y, t;
  scanf("%d", &n);
  for (i = 0; i < n; i++) scanf("%s", s[i]);
  for (i = 1; i < 21; i++) {
    if (found) break;
    f(i, 0);
  }
  printf("%s", st);
  return 0;
}
