#include <bits/stdc++.h>
using namespace std;
const long double eps = 1e-13;
const long double PI = acos(-1);
const int INF = (int)1e9 + 7;
const long long INFF = (long long)2e18;
const int mod = (int)1e9 + 7;
const int MXN = (int)1e6 + 7;
char buf[MXN];
int main() {
  scanf("%s", buf);
  char cur = 'a';
  for (int i = 0; buf[i]; i++) {
    if (buf[i] > cur) {
      puts("NO");
      return 0;
    } else if (buf[i] == cur) {
      cur++;
    }
  }
  puts("YES");
  return 0;
}
