#include <bits/stdc++.h>
using namespace std;
char a[50][50];
int n;
bool sea1(char b) {
  for (int i = 1; i <= n; i++) {
    int l = strlen(a[i]);
    for (int j = 0; j < l; j++)
      if (a[i][j] == b) return 1;
  }
  return 0;
}
bool sea2(char b, char c) {
  for (int i = 1; i <= n; i++) {
    int l = strlen(a[i]);
    for (int j = 0; j < l - 1; j++)
      if (a[i][j] == b && a[i][j + 1] == c) return 1;
  }
  return 0;
}
int main() {
  cin >> n;
  for (int i = 1; i <= n; i++) cin >> a[i];
  for (int i = 'a'; i <= 'z'; i++) {
    char temp = i;
    if (sea1(temp) == 0) {
      cout << temp << endl;
      return 0;
    }
  }
  for (int i = 'a'; i <= 'z'; i++)
    for (int j = 'a'; j <= 'z'; j++) {
      char b = i, c = j;
      if (sea2(b, c) == 0) {
        cout << b << c << endl;
        return 0;
      }
    }
}
