#include<bits/stdc++.h>
using namespace std;
int  main()
{int a,b,c,d;
 cin>>a>>b>>c>>d;
while((a>0) & (c>0))
{c=c-b;
 if(c<=0)
   cout<<"yes";
   break;
 a=a-d;
}
 if(a<=0)
   cout<<"no";
 else 
   cout<<"yes";
 return 0;
} 