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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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
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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
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#include <bits/stdc++.h>
using namespace std;
int main() {
  int n;
  cin >> n;
  string t[n];
  string ch = "";
  for (int i = 0; i < n; i++) {
    cin >> t[i];
    ch += t[i];
  }
  char x, l = '1';
  for (int m = 0; m < 26; m++) {
    x = 'a' + m;
    int c;
    c = int(ch.find(x));
    if (c == -1) {
      l = x;
      break;
    }
  }
  if (l != '1') {
    cout << l;
  } else {
    for (int d = 0; d < n; d++) {
      ch = t[d];
      string sh = "aa";
      char y, z;
      int w = 0;
      for (int j = 0; j < 26; j++) {
        sh[0] = 'a' + j;
        for (int k = 0; k < 26; k++) {
          sh[1] = 'a' + k;
          int c;
          c = int(ch.find(sh));
          if (c == -1) {
            w = 1;
            break;
          }
        }
        if (w == 1) {
          cout << sh;
          return 0;
        }
      }
      return 0;
    }
  }
}
