#include <bits/stdc++.h>
using namespace std;
int main() {
  vector<string> titles;
  int n;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    string title;
    cin >> title;
    titles.push_back(title);
  }
  for (char a = 'a'; a <= 'z'; ++a) {
    int found = false;
    for (int i = 0; i < titles.size() && !found; ++i) {
      if (titles[i].find(string() + a) != -1) found = true;
    }
    if (!found) {
      cout << string() + a;
      return 0;
    }
  }
  for (char a = 'a'; a <= 'z'; ++a) {
    for (char b = 'a'; b <= 'z'; ++b) {
      int found = false;
      for (int i = 0; i < titles.size() && !found; ++i) {
        if (titles[i].find(string() + a + b) != -1) found = true;
      }
      if (!found) {
        cout << string() + a + b;
        return 0;
      }
    }
  }
  return 0;
}
