3
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 1
z
 30
b
u
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f
m
t
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j
k
p
h
a
c
w
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z
p
t
d
o
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y
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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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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
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 30
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 30
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 1
b

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

#include <bits/stdc++.h>
using namespace std;
int main() {
  int n, m, x = 0;
  cin >> n;
  vector<string> v;
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    v.push_back(s);
  }
  static int a[27];
  for (int i = 0; i < n; i++) {
    string s = v[i];
    for (int j = 0; j < s.size(); j++) {
      int z = (int)(s[j] - 97);
      a[z]++;
    }
  }
  for (int j = 0; j < 26; j++) {
    if (a[j] == 0) {
      cout << (char)(97 + j) << endl;
      return 0;
    }
  }
  for (int i = 0; i < n; i++) {
    string s = v[i];
    for (int j = 97; j <= 122; j++) {
      string t;
      t = t + (char)j;
      t = t + 'a';
      for (int k = 97; k <= 122; k++) {
        t[1] = (char)k;
        int d = (int)s.find(t);
        if (d == (-1)) {
          cout << t << endl;
          return 0;
        }
      }
    }
  }
  return 0;
}
