#include <bits/stdc++.h>
using namespace std;
int main() {
  string str;
  cin >> str;
  int temp = 0;
  for (int i = 0; i < str.size(); i++) {
    if (str[i] - 'a' > temp)
      return cout << "NO\n", 0;
    else if (str[i] - 'a' == temp)
      temp++;
  }
  return cout << "YES\n", 0;
}
