#include <bits/stdc++.h>
int main() {
  std::string input;
  std::cin >> input;
  std::map<int, int> first_place;
  for (size_t i = 0; i < 26; ++i) {
    first_place[i] = 0;
  }
  for (size_t i = 0; i < input.size(); ++i) {
    if (first_place[input[i] - 'a'] == 0) first_place[input[i] - 'a'] = i + 1;
  }
  bool if_obf = true;
  for (size_t i = 0; i < first_place.size() - 1; ++i) {
    if ((first_place[i] > first_place[i + 1]) && (first_place[i + 1] != 0)) {
      if_obf = false;
      break;
    }
  }
  for (size_t i = 0; i < first_place.size() - 1; ++i) {
    if ((first_place[i] == 0) && (first_place[i + 1] != 0)) {
      if_obf = false;
      break;
    }
  }
  if (if_obf) {
    std::cout << "YES";
  } else {
    std::cout << "NO";
  }
}
