#include <bits/stdc++.h>
#pragma comment(linker, "/STACK:1024000000,1024000000")
using namespace std;
const double pi = acos(-1);
const double e = exp(1);
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
template <typename T>
inline void read(T &x) {
  T ans = 0;
  char last = ' ', ch = getchar();
  while (ch < '0' || ch > '9') last = ch, ch = getchar();
  while (ch >= '0' && ch <= '9') ans = ans * 10 + ch - '0', ch = getchar();
  if (last == '-') ans = -ans;
  x = ans;
}
const int maxn = 1000 + 10;
char str[maxn];
int vis[maxn];
int main() {
  while (~scanf("%s", str)) {
    memset(vis, 0, sizeof(vis));
    int flag = 1;
    for (int i = 0; str[i]; i++) {
      for (int j = 0; j < str[i] - 'a'; j++) {
        if (!vis[j]) {
          flag = 0;
          break;
        }
      }
      if (!flag) break;
      vis[str[i] - 'a'] = 1;
    }
    puts(flag ? "YES" : "NO");
  }
  return 0;
}
