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 30
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 2
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 5
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 1
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 10
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#include <bits/stdc++.h>
using namespace std;
int main() {
  int flag1[1000] = {0};
  int flag2[1000] = {0};
  int n;
  string str[50];
  cin >> n;
  for (int i = 0; i < n; i++) cin >> str[i];
  for (int i = 0; i < n; i++) {
    int j;
    for (j = 0; j < str[i].size() - 1; j++) {
      int t = (str[i][j] - 'a') * 26 + str[i][j + 1] - 'a';
      flag2[t] = 1;
      flag1[str[i][j] - 'a'] = 1;
    }
    flag1[str[i][j] - 'a'] = 1;
  }
  int k;
  for (k = 0; k < 26; k++) {
    if (!flag1[k]) break;
  }
  if (k < 26)
    cout << (char)('a' + k) << endl;
  else {
    for (int i = 0; i < 26 * 26; i++) {
      if (!flag2[i]) {
        cout << (char)('a' + i / 26) << (char)('a' + i % 26) << endl;
        break;
      }
    }
  }
  return 0;
}
