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