#include <bits/stdc++.h>
using namespace std;
template <class T>
inline void read(T &num) {
  T x = 0, f = 1;
  char ch = getchar();
  while (!isdigit(ch)) {
    if (ch == '-') f = -1;
    ch = getchar();
  }
  while (isdigit(ch)) {
    x = (x << 3) + (x << 1) + ch - '0';
    ch = getchar();
  }
  num = f * x;
}
int n;
map<string, int> s, t;
char c[505];
string hm;
int num;
bool vis[30];
int main() {
  scanf("%s", c + 1);
  int len = strlen(c + 1);
  char mn = 'a';
  for (int i = 1; i <= len; i++) {
    if (c[i] == mn) {
      mn++;
    } else if (c[i] > mn) {
      printf("NO");
      return 0;
    }
  }
  printf("YES");
}
