#include <bits/stdc++.h>
using namespace std;
void next(string &x) {
  for (int i = x.size() - 1; i >= 0; --i) {
    if (x[i] != 'z') {
      x[i] = (char)((int)x[i] + 1);
      for (int j = i + 1; j < x.size(); ++j) {
        x[j] = 'a';
      }
      return;
    }
  }
  x = 'a' + x;
  for (int i = 1; i < x.size(); ++i) {
    x[i] = 'a';
  }
}
int main() {
  int n;
  cin >> n;
  string data[n];
  for (int i = 0; i < n; ++i) {
    cin >> data[i];
  }
  string res = "a";
  bool fl = 0;
  while (!fl) {
    fl = 1;
    for (int i = 0; i < n; ++i) {
      for (int j = 0; j < data[i].size() - res.size() + 1; ++j) {
        for (int k = 0; k < res.size(); ++k) {
          if (data[i][j + k] != res[k]) {
            break;
          }
          if (k == res.size() - 1) {
            fl = 0;
          }
        }
      }
    }
    if (fl) {
      break;
    }
    next(res);
  }
  cout << res << endl;
  return 0;
}
