#include<stdio.h>

int kekka (int atk,int hp);

int main(){
  int a,b,c,d;
  scanf("%d %d %d %d",&a,&b,&c,&d);
  if( kekka(b,c) <= kekka(d,a) ){
    printf("yes\n");
  }else{
    printf("no\n");
  }
  return 0;
}

int kekka (int atk,int hp){
  return (hp + atk-1) / atk;
}