#include <bits/stdc++.h>
using namespace std;
int main() {
  ios::sync_with_stdio(false);
  cin.tie(NULL);
  cout.tie(NULL);
  long long int n;
  cin >> n;
  set<string> subv;
  for (long long int i = 0; i < n; i += 1) {
    string s;
    cin >> s;
    long long int len = s.size();
    for (long long int j = 0; j < len; j += 1) {
      for (long long int k = j; k < len; k += 1) {
        string temp = "";
        for (long long int start = j; start <= k; start++) {
          temp += s[start];
        }
        subv.insert(temp);
      }
    }
  }
  for (char a = 'a'; a <= 'z'; a++) {
    string temp = "";
    temp += a;
    if (!subv.count(temp)) {
      cout << temp;
      return 0;
    }
  }
  for (char a = 'a'; a <= 'z'; a++) {
    for (char b = 'a'; b <= 'z'; b++) {
      string temp = "";
      temp += a;
      temp += b;
      if (!subv.count(temp)) {
        cout << temp;
        return 0;
      }
    }
  }
  for (char a = 'a'; a <= 'z'; a++) {
    for (char b = 'a'; b <= 'z'; b++) {
      for (char c = 'a'; c <= 'z'; c++) {
        string temp = "";
        temp += a;
        temp += b;
        temp += c;
        if (!subv.count(temp)) {
          cout << temp;
          return 0;
        }
      }
    }
  }
  return 0;
}
