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#include <bits/stdc++.h>
using namespace std;
int n;
bool tag[26];
string s[31];
struct node {
  char a, b;
} num[26 * 26 + 5];
bool cmp(struct node x, struct node y) {
  if (x.a == y.a) return x.b < y.b;
  return x.a < y.a;
}
void solve() {
  for (int i = 0; i < 26; i++) {
    for (int j = 0; j < 26; j++) {
      num[i * 26 + j].a = i + 'a';
      num[i * 26 + j].b = j + 'a';
    }
  }
  sort(num, num + 26 * 26, cmp);
  for (int i = 0; i <= 26 * 26; i++) {
    int flag = 1;
    for (int j = 0; j < n; j++) {
      for (int k = 1; k < s[j].length(); k++) {
        if (s[j][k - 1] == num[i].a && s[j][k] == num[i].b) {
          flag = 0;
          break;
        }
      }
      if (flag == 0) break;
    }
    if (flag) {
      cout << num[i].a << num[i].b << endl;
      break;
    }
  }
}
int main() {
  while (cin >> n) {
    for (int i = 0; i < n; i++) {
      cin >> s[i];
      for (int j = 0; j < s[i].length(); j++) tag[s[i][j] - 'a'] = 1;
    }
    int flag = 1;
    for (int i = 0; i < 26; i++)
      if (tag[i] == 0) {
        cout << (char)(i + 'a') << endl;
        flag = 0;
        break;
      }
    if (flag) solve();
  }
  return 0;
}
