#include <bits/stdc++.h>
using namespace std;
const double pi = acos(-1);
bool comp(int i, int j) { return i > j; }
int main() {
  int n;
  string second;
  char start = 'a';
  cin >> second;
  for (long long i = 0; i < second.length(); ++i) {
    if (second[i] > start) {
      cout << "NO";
      return 0;
    } else if (second[i] == start)
      ++start;
  }
  cout << "YES";
  return 0;
}
