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