#include <bits/stdc++.h>
using namespace std;
const long double pi = 3.1415926535897932384626433832795l;
template <typename T>
inline auto sqr(T x) -> decltype(x * x) {
  return x * x;
}
template <typename T1, typename T2>
inline bool umx(T1& a, T2 b) {
  if (a < b) {
    a = b;
    return 1;
  }
  return 0;
}
template <typename T1, typename T2>
inline bool umn(T1& a, T2 b) {
  if (b < a) {
    a = b;
    return 1;
  }
  return 0;
}
bool use[26];
int main() {
  cout.setf(ios::showpoint | ios::fixed);
  cout.precision(20);
  string s;
  getline(cin, s);
  bool ok = 1;
  for (int i = int(0); i < int(((int)(s).size())); ++i) {
    int idx = s[i] - 'a';
    if (idx > 0) {
      if (!use[idx - 1]) {
        ok = 0;
      }
    }
    use[idx] = 1;
  }
  if (!ok) {
    puts("NO");
  } else {
    puts("YES");
  }
  return 0;
}
