#include <bits/stdc++.h>
using namespace std;
int main() {
  string s1;
  cin >> s1;
  long long int n = s1.size();
  string second = s1;
  sort(second.begin(), second.end());
  second.erase(unique(second.begin(), second.end()), second.end());
  long long int x = second.size();
  string p;
  for (long long int i = 0; i < x; i++) p.push_back(97 + i);
  if (p == second) {
    vector<long long int> v, w;
    for (long long int i = 0; i < x; i++) {
      for (long long int j = 0; j < n; j++) {
        if (p[i] == s1[j]) {
          v.push_back(j);
          break;
        }
      }
    }
    w = v;
    sort(w.begin(), w.end());
    if (v == w)
      cout << "YES";
    else
      cout << "NO";
  } else
    cout << "NO";
  return 0;
}
