#include <bits/stdc++.h>
const double EPS = 1e-24;
const int MOD = 1000000007ll;
const int mod1 = 1000000009ll;
const int mod2 = 1100000009ll;
const double PI = 3.14159265359;
long long INF = 1e18;
int inf = 1e9;
template <class T>
T Max2(T a, T b) {
  return a < b ? b : a;
}
template <class T>
T Min2(T a, T b) {
  return a < b ? a : b;
}
template <class T>
T Max3(T a, T b, T c) {
  return Max2(Max2(a, b), c);
}
template <class T>
T Min3(T a, T b, T c) {
  return Min2(Min2(a, b), c);
}
template <class T>
T Max4(T a, T b, T c, T d) {
  return Max2(Max2(a, b), Max2(c, d));
}
template <class T>
T Min4(T a, T b, T c, T d) {
  return Min2(Min2(a, b), Max2(c, d));
}
using namespace std;
int powermod(long long _a, long long _b, int _m = MOD) {
  long long _r = 1;
  while (_b) {
    if (_b % 2 == 1) _r = (_r * _a) % _m;
    _b /= 2;
    _a = (_a * _a) % _m;
  }
  return _r;
}
int add(int a, int b, int m = MOD) {
  int x = a + b;
  while (x >= m) x -= m;
  while (x < 0) x += m;
  return x;
}
int sub(int a, int b, int m = MOD) {
  int x = a - b;
  while (x < 0) x += m;
  while (x >= m) x -= m;
  return x;
}
int mul(int a, int b, int m = MOD) {
  long long x = a * 1ll * b;
  x %= m;
  while (x < 0) x += m;
  return (int)x;
}
long long gcd(long long a, long long b) {
  if (b == 0) return a;
  a %= b;
  return gcd(b, a);
}
int a[26];
int main() {
  std::ios::sync_with_stdio(false);
  cin.tie(0);
  string s;
  cin >> s;
  for (int i = 0; i < s.length(); i++) {
    a[s[i] - 'a']++;
  }
  int var = 0;
  for (int i = 0; i < s.length(); i++) {
    int t = s[i] - 'a';
    if (t > var) {
      cout << "NO\n";
      return 0;
    } else if (var == t) {
      var++;
      if (var == 27) var--;
    }
  }
  cout << "YES\n";
  return 0;
}
