#include <iostream>
using namespace std;

int main(){
	int a,b,c,d;
	int total,count;
	int i,n;

	cin>>a>>b>>c>>d;
	total=a+b;

	for(count=1;total<d && c*count<b && count<a;count++){
		total=c*count+count;
	}

	if(total==a+b){
		cout<<'0'<<endl;
		return 0;
	}

	cout<<count<<endl;
	return 0;
}