#include <bits/stdc++.h>

using namespace std;


int main() {
    double d,x,y,t;
    while(cin>>d>>x>>y>>t){
        double t1=d/x;
        double t2=d/y;
        if(t1+t<=t2)cout<<"YES"<<endl;
        else cout<<"NO"<<endl;
    }
}