#include<stdio.h>
int main(void){
  int A,B,C,D;
  scanf("%d %d %d %d",&A,&B,&C,&D);
  do{
    C=C-B;
    A=A-D;
  } while((C>0)&&(A>0));
  if((C<=0)&&(A>0)){
    printf("yes\n");
  }
  if((A<=0)&&(C>0)){
    printf("no\n");
  }
  return 0;
}