3
rjnflsbpxqivrcdjptj
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 1
z
 30
b
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n
f
m
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k
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c
w
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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
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 2
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 1
a
 30
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khbknqbvdkdd
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avptgpjmenfzfxwckmt
 30
e
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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;
int dx[] = {-1, -1, -1, 0, 0, 1, 1, 1};
int dy[] = {-1, 0, 1, -1, 1, -1, 0, 1};
int xChange[] = {0, 1, 0, -1};
int yChange[] = {1, 0, -1, 0};
const int N = 30;
int mark[26];
string a[N];
vector<char> vc;
vector<string> vs;
int main() {
  ios::sync_with_stdio(false), cin.tie(NULL);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    for (int j = 0; j < a[i].size(); j++) {
      int x = a[i][j] - 'a';
      if (!mark[x]) mark[x] = 1;
    }
  }
  for (int i = 0; i < 26; i++) {
    if (!mark[i]) {
      char x = 'a' + i;
      cout << x;
      return 0;
    }
  }
  for (int i = 0; i < n; i++) {
    string s = a[i];
    for (int j = 0; j < (int)s.size(); j++) {
      string d = "";
      d += s[j];
      for (int h = j + 1; h < (int)s.size(); h++) {
        d += s[h];
        vs.push_back(d);
      }
    }
  }
  sort(vs.begin(), vs.end());
  string ans = vs[0], temp = vs[0];
  for (int i = (int)ans.size() - 1; i >= 0; i--) {
    char x = ans[i];
    if (x - 'a' < 25) {
      x = x + 1;
      ans[i] = x;
      break;
    }
  }
  if (temp == ans) {
    ans.insert(0, 1, 'a');
  }
  cout << ans;
  return 0;
}
