#include <bits/stdc++.h>
using namespace std;
map<int, int> mapp;
vector<pair<int, int> > otr;
vector<int> g[1000005];
int w[1000005];
int main() {
  bool al[27] = {false};
  string s;
  cin >> s;
  char li = 'a';
  for (int i = 0; i < s.length(); i++) {
    if (s[i] > li) {
      cout << "NO";
      return 0;
    }
    if (s[i] == li) li++;
  }
  cout << "YES";
}
