#include <bits/stdc++.h>
using namespace std;

main(){
    int A,B,C,D;
    cin >> A >> B >> C >> D;
  while(!(A<=0 || C<=0)){
    A=A-D;
    C=C-B;
  }

if(A<=0 && C<=0){
    cout << "YES" <<endl;
  }else if(C<=0){
    cout << "NO" <<endl;
  }else{
    cout << "YES" <<endl;
  }
}