#include <iostream>
#include <cmath>

using namespace std;

int R0, W0, C, R;

int main() {
	while (cin >> R0 >> W0 >> C >> R && R) {
		if (R0 >= C*W0) {
			cout << 0 << endl;
		} else {
			cout << ceil((C*W0-R0)/((double)R))<< endl;
		}
	}
}