3
rjnflsbpxqivrcdjptj
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 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
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abcdefghijklmnopqrst
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 2
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 5
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abcdefghijklmnopqrst
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 1
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 10
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 5
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abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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ge
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 1
b

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

#include <bits/stdc++.h>
using namespace std;
int d[3023];
int main() {
  int n;
  cin >> n;
  char f[32][25];
  for (int i = 0; i < n; i++) {
    cin >> f[i];
  }
  char a = 'a';
  char b = 'a';
  int flag = 0;
  for (int y = 0; y < 26; y++) {
    flag = 1;
    for (int i = 0; i < n; i++) {
      for (int j = 0; j < strlen(f[i]); j++) {
        if (f[i][j] == a) {
          flag = 0;
          break;
        }
      }
    }
    if (flag) {
      cout << a;
      break;
    }
    a++;
  }
  if (!flag) {
    a = 'a';
    b = 'a';
    for (int z = 0; z < 26; z++) {
      b = 'a';
      for (int y = 0; y < 26; y++) {
        flag = 1;
        for (int i = 0; i < n; i++) {
          for (int j = 0; j < strlen(f[i]) - 1; j++) {
            if (f[i][j] == a && f[i][j + 1] == b) {
              flag = 0;
              break;
            }
          }
        }
        if (!flag) b++;
      }
      if (flag) {
        cout << a << b;
        break;
      }
      a++;
    }
  }
  return 0;
}
