#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[1000007];
  map<char, long long int> mp;
  cin >> s;
  long long int l = strlen(s), i;
  mp[s[0]] = 1;
  if (s[0] != 'a') {
    cout << "NO";
    return 0;
  }
  for (i = 1; i < l; i++) {
    if ((s[i - 1] > s[i] && mp[s[i]] == 0) ||
        mp[s[i] - 1] == 0 && s[i] != 'a') {
      cout << "NO";
      return 0;
    } else
      mp[s[i]] = 1;
  }
  cout << "YES";
  return 0;
}
