#include<stdio.h>

int main(){

  int a,b,c,d;

  scanf("%d %d %d %d", &a,&b,&c,&d);

while(a > 0){
  c-=b;
  if(c <= 0) break;
  a-=d;
}
if(c > 0) printf("No\n");
else printf("yes\n");

  return 0;
}