3
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 1
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 30
b
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 5
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 2
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 5
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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
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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
 b
 aa
 l
 a

#include <bits/stdc++.h>
using namespace std;
unordered_set<string> st;
void rec(string& cur) {
  ;
  if ((int)cur.size() > 0 && !st.count(cur)) {
    cout << cur << '\n';
    exit(0);
  }
  if ((int)cur.size() > 6) {
    return;
  }
  string tmp = cur;
  for (char c = 'a'; c <= 'z'; c++) {
    tmp.push_back(c);
    if ((int)tmp.size() > 0 && !st.count(tmp)) {
      cout << tmp << '\n';
      exit(0);
    }
    tmp.pop_back();
  }
  for (char c = 'a'; c <= 'z'; c++) {
    cur.push_back(c);
    rec(cur);
    cur.pop_back();
  }
}
void solve() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    string s;
    cin >> s;
    int m = s.size();
    for (int j = 0; j < m; j++) {
      for (int k = j; k < m; k++) {
        st.insert(s.substr(j, k - j + 1));
      }
    }
  }
  string ans;
  rec(ans);
  assert(false);
}
int main() {
  ios_base::sync_with_stdio(0);
  cin.tie(0);
  solve();
  return 0;
}
