#include<iostream>
#include<algorithm>
using namespace std;
int main(){
	int r,w,c,d;
	while(true){
	cin>>r>>w>>c>>d;
	if(!(r+w+c+d))break;
	w=w*c-r;
	if(w<=0){cout<<"0"<<endl;goto loop;}
	if(w%d==0)cout<<w/d<<endl;
	else cout<<w/d+1<<endl;
	loop:;
	}
	return 0;
}