3
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 1
z
 30
b
u
p
n
f
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j
k
p
h
a
c
w
z
z
p
t
d
o
w
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q
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 5
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 2
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 5
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abcdefghijklmnopqrst
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 2
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 5
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 1
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 10
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 5
dfijdfhi
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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xcn
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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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 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 int MX = 35;
static int nodes = 0;
int n, trie[2123456][26];
string s[MX];
void Insert(int st, int id) {
  int cur = 0;
  for (int i = st; s[id][i]; i++) {
    if (trie[cur][s[id][i] - 'a'] == -1) {
      trie[cur][s[id][i] - 'a'] = ++nodes;
      for (int i = 0; i < 26; i++) trie[nodes][i] = -1;
    }
    cur = trie[cur][s[id][i] - 'a'];
  }
}
string customSearch() {
  string ret = "";
  int cur = 0;
  while (~cur) {
    for (int i = 0; i < 26; i++) {
      if (trie[cur][i] == -1) {
        ret += ('a' + i);
        return ret;
      }
    }
    for (int i = 0; i < 26; i++) {
      if (trie[cur][i] == -1) continue;
      ret += ('a' + i);
      cur = trie[cur][i];
      break;
    }
  }
  ret += 'a';
  return ret;
}
int main() {
  ios_base ::sync_with_stdio(false);
  cin.tie(NULL);
  cin >> n;
  for (int i = 0; i < n; i++) cin >> s[i];
  for (int i = 0; i < 26; i++) trie[0][i] = -1;
  for (int i = 0; i < n; i++) {
    for (int j = 0; s[i][j]; j++) {
      Insert(j, i);
    }
  }
  string res = customSearch();
  cout << res << endl;
  return 0;
}
