#include <bits/stdc++.h>
using namespace std;
bool Visited[600];
int main() {
  memset(Visited, false, 600);
  string A;
  cin >> A;
  if (int(A[0] == 97)) {
    Visited[97] = true;
  } else if (int(A[0]) != 97) {
    cout << "NO" << endl;
    return 0;
  }
  for (int i = 1; i < A.size(); i++) {
    if (!Visited[int(A[i] - 1)] && !Visited[int(A[i])]) {
      cout << "NO" << endl;
      return 0;
    }
    Visited[int(A[i])] = true;
  }
  cout << "YES" << endl;
}
