#include <bits/stdc++.h>
using namespace std;
int dx[] = {-1, -1, -1, 0, 0, 1, 1, 1};
int dy[] = {-1, 0, 1, -1, 1, -1, 0, 1};
int xChange[] = {0, 1, 0, -1};
int yChange[] = {1, 0, -1, 0};
const int N = 30;
int mark[26];
string a[N];
set<string> se;
int main() {
  ios::sync_with_stdio(false), cin.tie(NULL);
  int n;
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> a[i];
    for (int j = 0; j < a[i].size(); j++) {
      int x = a[i][j] - 'a';
      if (!mark[x]) mark[x] = 1;
    }
  }
  for (int i = 0; i < 26; i++) {
    if (!mark[i]) {
      char x = 'a' + i;
      cout << x;
      return 0;
    }
  }
  for (int i = 0; i < n; i++) {
    string s = a[i];
    for (int j = 0; j < (int)s.size(); j++) {
      string d = "";
      d += s[j];
      for (int h = j + 1; h < (int)s.size(); h++) {
        d += s[h];
        se.insert(d);
      }
    }
  }
  set<string>::iterator it = se.begin();
  string ans = *it, temp = *it;
  for (int i = 0; i < 26; i++) {
    for (int j = 0; j < 26; j++) {
      string dd = "";
      dd += i + 'a';
      dd += j + 'a';
      set<string>::iterator it = se.find(dd);
      if (it == se.end()) {
        cout << dd;
        return 0;
      }
    }
  }
  return 0;
}
