#include <bits/stdc++.h>
using namespace std;
const int MAX_N = 100005;
int main() {
  cin.tie(0);
  ios::sync_with_stdio(false);
  string s;
  cin >> s;
  int n = (int)(s).size();
  int st = (int)(s[0] - 'a');
  vector<string> ss;
  string nw;
  nw.push_back(s[0]);
  for (int i = (int)(1); i < (int)(n); ++i) {
    if ((int)(s[i] - 'a') < st) {
      cout << "No\n";
      return 0;
    } else if ((int)(s[i] - 'a') == st) {
      ss.push_back(nw);
      nw.clear();
      nw.push_back(s[0]);
    } else {
      nw.push_back(s[i]);
    }
  }
  ss.push_back(nw);
  string pos = ss.back();
  for (int i = (int)((int)(ss).size() - 1) - 1; i >= 0; --i) {
    if (ss[i] > pos) {
      cout << "No\n";
      return 0;
    } else {
      pos = ss[i] + pos;
    }
  }
  cout << "Yes\n";
  return 0;
}
