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

int main(int argc, char const *argv[])
{
	int a,b,c,d;
	while(cin>>a>>b>>c>>d,a)
	{
		int ans=0;
		while(a/b<c) a+=d,++ans;
		cout<<ans<<endl;
	}
	return 0;
}
