#include<iostream>
using namespace std;
int main(){
	while(1){
		double r,w,C,R;
		cin >> r >> w >> C >> R;
		if(!r&&!w&&!C&&!R)	break;
		if(r>=C*w)	cout << 0 << endl;
		else{
			cout << int((C*w-r)/R+0.999) << endl;
		}
	}
}
