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 1
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 30
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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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 2
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 1
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 30
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 30
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 1
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 e
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 aa
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 ab
 z
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 l
 a

#include <bits/stdc++.h>
using namespace std;
vector<string> v(40, "");
string sl;
int n;
bool check(string st) {
  int l, l1;
  l = st.length();
  for (int i = 0; i < n; i++) {
    l1 = v[i].length();
    for (int j = 0; j < l1 - l + 1; j++) {
      if (st[0] == v[i][j]) {
        for (int k = 0; k < l; k++)
          if (st[k] != v[i][j + k]) return true;
        return false;
      }
    }
  }
  return true;
}
int main() {
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> v[i];
  }
  sl = "";
  for (int i1 = 0; i1 < 26; i1++) {
    sl = char(i1 + 97);
    if (check(sl)) {
      cout << sl;
      return 0;
    }
  }
  for (int i1 = 0; i1 < 26; i1++)
    for (int i2 = 0; i2 < 26; i2++) {
      sl = char(i1 + 97);
      sl += char(i2 + 97);
      if (check(sl)) {
        cout << sl;
        return 0;
      }
    }
  for (int i1 = 0; i1 < 26; i1++)
    for (int i2 = 0; i2 < 26; i2++)
      for (int i3 = 0; i3 < 26; i3++) {
        sl = char(i1 + 97);
        sl += char(i2 + 97);
        sl += char(i3 + 97);
        if (check(sl)) {
          cout << sl;
          return 0;
        }
      }
  for (int i1 = 0; i1 < 26; i1++)
    for (int i2 = 0; i2 < 26; i2++)
      for (int i3 = 0; i3 < 26; i3++)
        for (int i4 = 0; i4 < 26; i4++) {
          sl = char(i1 + 97);
          sl += char(i2 + 97);
          sl += char(i3 + 97);
          sl += char(i4 + 97);
          if (check(sl)) {
            cout << sl;
            return 0;
          }
        }
  for (int i1 = 0; i1 < 26; i1++)
    for (int i2 = 0; i2 < 26; i2++)
      for (int i3 = 0; i3 < 26; i3++)
        for (int i4 = 0; i4 < 26; i4++)
          for (int i5 = 0; i5 < 26; i4++) {
            sl = char(i1 + 97);
            sl += char(i2 + 97);
            sl += char(i3 + 97);
            sl += char(i4 + 97);
            sl += char(i5 + 97);
            if (check(sl)) {
              cout << sl;
              return 0;
            }
          }
  cout << int('a');
  return 0;
}
