#include <iostream>
using namespace std;
const int N = 27;
char in[N]="abcdefghijklmnopqrstuvwxyz";
int main(){
char out[N];
while(cin>>out){
char stk[N],tp=-1;
int j = 0;
for(int i=0;i<=25;i++){
stk[++tp] = in[i];
while(tp != -1 && stk[tp] == out[j]){
--tp;
j++;
}
}
if(tp == -1) cout<<"yes"<<endl;
else cout<<"no"<<endl;
}
return 0;
}