#include <bits/stdc++.h>
using namespace std;
const int maxn = 1e6+100;
char a[1005];
int vis[maxn];
int main()
{
    scanf("%s",a);
    int n = strlen(a);
    int flag = 0;
    char ans = 'a';
    vis[0] = 1;
    if(a[0]!='a')
        flag = 1;
    for(int i=0;i<n;i++)
    {
        if(!vis[a[i]-'a'])
        {
            if(a[i]==ans+1)
                ans++;
            else
            {
                flag = 1;
                break;
            }
        }
        vis[a[i]-'a'] = 1;
    }
    if(flag)
        puts("NO");
    else
        puts("YES");
    return 0;
}