#include <bits/stdc++.h>
using namespace std;
int main() {
  int len, i;
  bool chk = true;
  string str;
  cin >> str;
  char ch = 'a';
  len = str.size();
  for (i = 0; i < len; i++) {
    if (str[i] > ch) {
      chk = false;
      break;
    } else if (str[i] == ch)
      ch++;
  }
  chk ? printf("YES\n") : printf("NO\n");
  return 0;
}
