#include <bits/stdc++.h>
using namespace std;
int main() {
  char cnt = '`';
  string s;
  cin >> s;
  while (s.size()) {
    if (s[0] != cnt + 1) {
      cout << "NO";
      return 0;
    }
    while (s.find(cnt + 1) != string::npos) {
      s.erase(s.find(cnt + 1), 1);
    }
    cnt++;
  }
  cout << "YES";
  return 0;
}
