3
rjnflsbpxqivrcdjptj
vpojopbwbwbswdu
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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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vgnsdfdgfh
 2
rxscdzkkezud
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 5
ojdfhi
abcdefghijklmnopqrst
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qqq
 1
epkotfpkkrhhmuipmtdk
 10
kpmwcdoysw
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 5
dfijdfhi
abcdefghijklmnopqrst
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 2
xlaxwpjabtpwddc
txwdjmohrrszorrnomc
 1
a
 30
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bzlnmd
xpbtbpftimnn
khbknqbvdkdd
mldj
skrvnyz
avptgpjmenfzfxwckmt
 30
e
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x
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ge
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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;
set<string> s;
char _s[6];
int main() {
  int N;
  string sub;
  scanf(" %d", &N);
  for (int i = 0; i < N; i++) {
    cin >> sub;
    for (int j = 0; j < sub.size(); j++) {
      for (int k = j; k < sub.size() && k < j + 4; k++) {
        s.insert(sub.substr(j, k - j + 1));
      }
    }
  }
  sub = "";
  for (_s[0] = 'a'; _s[0] <= 'z'; _s[0]++) {
    if (s.find(string(_s)) == s.end()) {
      cout << string(_s) << endl;
      return 0;
    }
  }
  for (_s[0] = 'a'; _s[0] <= 'z'; _s[0]++) {
    for (_s[1] = 'a'; _s[1] <= 'z'; _s[1]++) {
      if (s.find(string(_s)) == s.end()) {
        cout << string(_s) << endl;
        return 0;
      }
    }
  }
  for (_s[0] = 'a'; _s[0] <= 'z'; _s[0]++) {
    for (_s[1] = 'a'; _s[1] <= 'z'; _s[1]++) {
      for (_s[2] = 'a'; _s[2] <= 'z'; _s[2]++) {
        if (s.find(string(_s)) == s.end()) {
          cout << string(_s) << endl;
          return 0;
        }
      }
    }
  }
  for (_s[0] = 'a'; _s[0] <= 'z'; _s[0]++) {
    for (_s[1] = 'a'; _s[1] <= 'z'; _s[1]++) {
      for (_s[2] = 'a'; _s[2] <= 'z'; _s[2]++) {
        for (_s[3] = 'a'; _s[3] <= 'z'; _s[3]++) {
          if (s.find(string(_s)) == s.end()) {
            cout << string(_s) << endl;
            return 0;
          }
        }
      }
    }
  }
  return 0;
}
