#include <bits/stdc++.h>
using namespace std;
int i, j, n, k, l, m, tot, flag, r, z, K, fldgjdflgjhrthrl, fldggfhfghjdflgjl,
    fldgjdflgrtyrtyjl, ffgfldgjdflgjl, x3, y3, x, y, h, num, h2, timer, sz, q,
    c;
string s, t;
int used[500];
int main() {
  cin >> s;
  n = s.size();
  for (i = 0; i < n; i++)
    if (!used[s[i]]) {
      used[s[i]] = 1;
      t.push_back(s[i]);
    }
  m = t.size();
  for (i = 0; i < m; i++)
    if (t[i] != i + 'a') {
      cout << "NO" << endl;
      return 0;
    }
  cout << "YES" << endl;
  return 0;
}
