#include <bits/stdc++.h>
using namespace std;
int main() {
  char str[1000], ch, check[10000], big;
  int arr[50000], flag = 0;
  cin >> str;
  int l = strlen(str);
  if (str[0] == 'a') {
    big = str[0];
    for (int i = 1; i < l; i++) {
      if (str[i] > big) {
        if (str[i] != big + 1) {
          flag = 1;
          break;
        } else {
          flag = 0;
          big = str[i];
        }
      }
    }
    if (flag == 0)
      cout << "YES" << endl;
    else
      cout << "NO" << endl;
  } else {
    cout << "NO" << endl;
  }
  return 0;
}
