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#include <bits/stdc++.h>
using namespace std;
typedef struct node {
  node *x[26];
  node() {
    for (int i = 0; i < 26; ++i) x[i] = NULL;
  }
} node;
node *root;
void add(string s) {
  node *temp = root;
  int len = s.size();
  for (int i = 0; i < len; ++i) {
    if (temp->x[s[i] - 'a'] == NULL) {
      temp->x[s[i] - 'a'] = new node;
    }
    temp = temp->x[s[i] - 'a'];
  }
}
string solve() {
  string ans = "";
  node *temp = root;
  while (1) {
    for (int i = 0; i < 26; ++i) {
      if (temp->x[i] == NULL) {
        ans += 'a' + i;
        return ans;
      }
    }
    ans += 'a';
    temp = temp->x[0];
  }
}
int main() {
  root = new node;
  int n;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    string s;
    cin >> s;
    int len = s.size();
    for (int i = 0; i < len; ++i) {
      string temp = s.substr(i, len);
      add(temp);
    }
    add(s);
  }
  cout << solve() << endl;
}
