#include <bits/stdc++.h>
using namespace std;
string s;
int i;
int letters;
bool possible;
int main() {
  letters = 1;
  i = 0;
  possible = true;
  getline(cin, s);
  while ((s[i] > 96) && (s[i] < 123)) {
    if ((s[i] > 96) && (s[i] < 123)) {
      if ((s[i] > (96 + letters)) && (s[i] < 123)) {
        possible = false;
      }
      if (s[i] == (96 + letters)) {
        letters++;
      }
    }
    i++;
  }
  if (possible) {
    cout << "YES";
  } else {
    cout << "NO";
  }
  return 0;
}
