#include <bits/stdc++.h>
using namespace std;
int main() {
  char s[510];
  int i, f[26] = {0}, ans = 0, flag = 0;
  scanf("%s", s);
  int l = strlen(s);
  for (i = 0; i < l; i++) {
    if (f[s[i] - 'a'] || (s[i] - 'a') == ans) {
      if (f[s[i] - 'a'] == 0) ans++;
      f[s[i] - 'a'] = 1;
    } else {
      flag = 1;
      break;
    }
  }
  if (flag)
    printf("NO\n");
  else
    printf("YES\n");
  return 0;
}
