#include <bits/stdc++.h>
using namespace std;
int b[26];
int main() {
  int i, j, k, l, n;
  cin >> n;
  string a[n];
  for (i = 0; i < n; i++) {
    cin >> a[i];
  }
  for (i = 0; i < n; i++) {
    for (j = 0; j < a[i].size(); j++) b[a[i][j] - 97]++;
  }
  for (i = 0; i < 26; i++)
    if (b[i] == 0) {
      cout << (char)(i + 97);
      return 0;
    }
  for (i = 0; i < 26; i++)
    for (j = 0; j < 26; j++) {
      char x = i + 97;
      char y = j + 97;
      int flag = 0;
      for (k = 0; k < n; k++) {
        int m = 0;
        while (a[k][m] != '\0') {
          if (a[k][m] == x && a[k][m + 1] == y) {
            flag = 1;
            break;
          }
          m++;
        }
      }
      if (flag == 0) {
        cout << x << y;
        return 0;
      }
    }
}
