#include <bits/stdc++.h>
using namespace std;
int main() {
  string theString;
  cin >> theString;
  bool wrong = false;
  for (int i = 0; i < 26; ++i) {
    string tempString = "";
    if (theString.length() != 0) {
      if (theString[0] - 'a' == i) {
        for (int j = 0; j < theString.length(); ++j) {
          if (theString[j] - 'a' != i) tempString += theString[j];
        }
      } else {
        wrong = true;
        break;
      }
      theString = tempString;
    }
  }
  if (wrong) {
    cout << "NO" << endl;
  } else {
    cout << "YES" << endl;
  }
  return 0;
}
