text: /****************************************************************************************
  *  @author: kzvd4729                                         created: Nov/20/2018 14:27                        
  *  solution_verdict: Wrong answer on test 5                  language: GNU C++14                               
  *  run_time: 15 ms                                           memory_used: 0 KB                                 
  *  problem: https://codeforces.com/contest/765/problem/B
  ****************************************************************************************/
  #include<bits/stdc++.h>
  #define long long long
  using namespace std;
  const int N=1e6;
  int main()
  {
    ios_base::sync_with_stdio(0);cin.tie(0);
    string s;cin>>s;
    char last='a';
    for(auto x:s)
    {
      if(x<=last)continue;
      last++;
      if(last!=x)cout<<"NO"<<endl,exit(0);
    }
    cout<<"YES"<<endl;
    return 0;
  }