#include<bits/stdc++.h>

using namespace std;

int func(int a, int b, int c, int d){
	for(int i=0;;++i){
		int current = a + i * d;
		if(current/c >= b)return i;
	}
}

int main(){
	int a, b, c, d;
	while(true){
		cin >> a >> b >> c >> d;
		if(!(a+b+c+d))break;
		cout << func(a, b, c, d) << endl;
	}

	return 0;
}

