#include <bits/stdc++.h>
using namespace std;
int main() {
  char str[510];
  int n, i;
  bool rec[27];
  while (cin >> str) {
    int flag = 0, faill = 0;
    memset(rec, 0, sizeof(rec));
    int t = strlen(str);
    if (str[0] != 'a') faill = 1;
    rec[1] = 1;
    rec[0] = 1;
    int sum = 1;
    for (i = 0; i < t; i++) {
      if (rec[str[i] - 'a' + 1] == 0) {
        rec[str[i] - 'a' + 1] = 1;
        for (int j = i - 1; j >= 1; j--) {
          if (str[j] > str[i]) {
            faill = 1;
            break;
          }
        }
      }
    }
    for (int i = 26; i >= 1; i--) {
      if (flag == 1 && rec[i] == 0) {
        faill = 1;
        break;
      }
      if (rec[i]) flag = 1;
    }
    if (faill)
      cout << "NO" << endl;
    else
      cout << "YES" << endl;
  }
  return 0;
}
