#include <bits/stdc++.h>
using namespace std;
char s[505];
int main() {
  scanf("%s", s);
  char nx = 'a';
  for (int i = 0; s[i]; i++) {
    if (s[i] == nx)
      nx++;
    else if (s[i] > nx) {
      printf("NO\n");
      return 0;
    }
  }
  printf("YES\n");
  return 0;
}
