#include<stdio.h>
int main(void)
{
	int a,b,c,d,t,o;
	scanf("%d %d %d %d",&a,&b,&c,&d);
	t=a;
	o=c;
	while(1){
		t=a-d;
		o=c-b;
		if(t<=0){
			printf("no\n");
			break;
		}
	}
		if(a<=0){
			printf("yes\n");
			break;
		}
	return 0;
}