#include<iostream>
#include<cstdio>
using namespace std;
int main(){
    double D,X,Y,T;
    while(scanf("%lf %lf %lf %lf",&D,&X,&Y,&T) != EOF)
        if(D/X + T < D/Y) printf("YES\n")
        else printf("NO\n");
    return 0;
}