#include <bits/stdc++.h>
using namespace std;
int main() {
  map<string, int> a;
  int n;
  cin >> n;
  for (int i = 1; i <= n; i++) {
    string q;
    cin >> q;
    for (int l = 0; l <= q.size() - 1; l++) {
      char p[50];
      for (int i = 0; i <= 49; i++) p[i] = 0;
      int o = 0;
      for (int j = l; j < q.size(); j++) {
        p[o++] = q[j];
        a[p] = 1;
      }
    }
  }
  char p[50];
  for (int i = 0; i <= 49; i++) p[i] = 0;
  int o = 0;
  int s = 0;
  while (1) {
    if (!s) {
      p[o] = 'a';
      if (!a.count(p)) {
        cout << p;
        return 0;
      }
      s = 1;
      continue;
    }
    p[o]++;
    int s = o;
    while (p[o] > 'z') {
      p[o] = 'a';
      o--;
      if (o < 0) {
        p[s + 1] = 'a';
        s++;
        break;
      } else if (o >= 0) {
        p[o]++;
      }
    }
    o = s;
    if (!a.count(p)) {
      cout << p;
      return 0;
    }
  }
}
