#include <iostream>
using namespace std;

int main() {
	int r0, w0, c, r;
	while (cin >> r0 >> w0 >> c >> r) {
		if (!r0) break;
		int i;
		for (i = 0; ; i++) {
			if ((r0 + r * i) >= w0 * c) break;
		}
		cout << i << endl;
	}
	return 0;
}