#include<stdio.h>
int main()
{
  long long int a,b,c,d;
  scanf("%lld%lld%lld%lld",&a,&b,&c,&d);
  while( c >0 || a>0)
  {
    c=c-b;
      if(c <=0)
      {printf("yes");
       break;
      }
    a=a-d;
      if(a<=0)
      {printf("no");
      break;}
  }
return 0;}