#include <bits/stdc++.h>
using namespace std;
int main() {
  int s, a, e = 1, p = 0, let = 0, f = 0;
  int l[26];
  for (int i = 0; i < 26; i++) l[i] = 0;
  string x;
  cin >> x;
  s = x.size();
  for (int i = 0; i < s; i++) {
    a = x[i] - 97;
    l[a]++;
  }
  for (int i = 0; i < 26; i++) {
    if (l[i] == 0)
      e = 0;
    else if ((l[i] > 0) && (e == 0)) {
      cout << "NO";
      exit(0);
    } else if (l[i] > 0)
      let++;
  }
  if (x[0] != 'a') {
    cout << "NO";
    exit(0);
  }
  for (int i = 0; i < let; i++) {
    f = 0;
    for (int j = 0; j < s; j++) {
      a = x[j] - 97;
      if (((a == i) && (j >= p)) && (f == 0)) {
        p = j;
        f++;
        break;
      }
      if ((a == i) && (j < p)) {
        cout << "NO";
        exit(0);
      }
    }
  }
  cout << "YES";
  return 0;
}
