#include<stdio.h>

int main(void){
  int A,B,C,D,i=1;

  scanf("%d %d %d %d",&A,&B,&C,&D);
  while(i!=0){
    C=C-B;
    if(C<=0)
      i=0;
    A=A-D;
    if(A<=0)
      i=0;
  }
  if(C<=0)
    printf("Yes\n");
  else if(A<=0)
    printf("No\n");
  else
    printf("error\n");
  
  return 0;
}
