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
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a
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 e
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 aa
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 ab
 z
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 a

#include <bits/stdc++.h>
using namespace std;
string s = "", t, c = "";
int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) cin >> t, s += t, s += " ";
  string s2 = "";
  while (1) {
    for (int i = 97; i < 123; i++) {
      c += char(i);
      int l = 0;
      for (int h = 0; h < s.length(); h++)
        if (s[h] == c[0]) {
          int k = 1;
          while (1) {
            if (k == c.length()) break;
            if (s[k + h] == c[k])
              k++;
            else
              break;
          }
          if (k == c.length()) {
            l = 1;
            break;
          }
        }
      if (l != 1)
        for (int h = 0; h < s.length(); h++)
          if (s[h] != c[0]) {
            int k = 1;
            while (1) {
              if (k == c.length()) break;
              if (s[k + h] != c[k])
                k++;
              else
                break;
            }
            if (k == c.length()) {
              cout << c << endl;
              return 0;
            }
          }
      c = s2;
    }
    s2 += "a";
  }
  return 0;
}
