#include <bits/stdc++.h>
using namespace std;
long long power(int x, unsigned int y) {
  long long res = 1;
  while (y > 0) {
    if (y & 1) res = res * x;
    y >>= 1;
    x *= x;
  }
  return res;
}
void solve() {
  string str;
  cin >> str;
  char ch = 'a';
  for (int i = 0; i < str.length(); i++) {
    if (str[i] == ch)
      ch++;
    else if (str[i] > ch) {
      cout << "NO";
      return;
    }
  }
  cout << "YES";
}
int main() {
  ios_base::sync_with_stdio(false);
  cin.tie(NULL);
  solve();
}
