#include <iostream>
using namespace std;
#define vi vector<int>



int main(){
  ios_base::sync_with_stdio(false);
  int a, b, c, d;
  cin>>a>>b>>c>>d;

  while(1){
  	c -= b;
  	if(c <= 0){
  		cout<<"YES"<<endl;
  		return 0;
  	}

  	a -= d;
  	if(a <= 0){
  		cout<<"NO"<<endl;
  		return 0;
  	}
  	
  }
}