#include <bits/stdc++.h>
char ss[30][21];
bool add1(char* s, int l) {
  int i = l - 1;
  s[i]++;
  while (s[i] > 'z') {
    s[i] = 'a';
    if (i == 0) return true;
    s[i - 1]++;
    i--;
  }
  return false;
}
int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; ++i) {
    scanf("%s", ss[i]);
  }
  char s[21];
  char aux[21];
  int l = 1;
  strcpy(s, "a");
  while (l < 21) {
    bool ok = true;
    for (int i = 0; i < n && ok; ++i) {
      ok = strstr(ss[i], s) == NULL;
    }
    if (ok) break;
    if (add1(s, l)) {
      l++;
      strcpy(aux, s);
      strcpy(s + 1, aux);
      s[0] = 'a';
    }
  }
  printf("%s\n", s);
  return 0;
}
