#include <bits/stdc++.h>
using namespace std;
int main() {
  string str;
  int i;
  cin >> str;
  char current = 'a';
  bool ans = true;
  if (str[0] != current) ans = false;
  for (i = 1; i < str.length(); i++) {
    if (str[i] <= current)
      continue;
    else if (str[i] == current + 1)
      current = str[i];
    else {
      ans = false;
      break;
    }
  }
  if (ans)
    cout << "YES";
  else
    cout << "NO";
  return 0;
}
