#include <bits/stdc++.h>
using namespace std;
struct point {
  int x, y;
};
bool Find(string a, string A[50], int n) {
  int can = 1, tool = 0;
  for (int k = 0; k < n; k++) {
    tool = A[k].length() - a.length();
    for (int i = 0; i <= tool; i++) {
      can = 1;
      for (int j = i; j < i + a.length(); j++)
        if (A[k][j] != a[j - i]) {
          can = 0;
          break;
        }
      if (can == 1) return true;
    }
  }
  return false;
}
int main() {
  string s = "abcdefghijklmnopqrstuvwxyz";
  int B[50] = {};
  int n;
  string A[50];
  cin >> n;
  for (int i = 0; i < n; i++) {
    cin >> A[i];
    for (int j = 0; j < A[i].length(); j++) B[s.rfind(A[i][j])] = 1;
  }
  for (int i = 0; i < 26; i++) {
    if (B[i] == 0) {
      cout << s[i] << endl;
      return 0;
    }
  }
  string d = "";
  for (int i = 0; i < 26; i++)
    for (int j = 0; j < 26; j++) {
      d = "";
      d.push_back(s[i]);
      d.push_back(s[j]);
      if (!Find(d, A, n)) {
        cout << d << endl;
        return 0;
      }
    }
  return 0;
}
