#include <iostream>

using namespace std;

int main()
{
    while (true) {
        int r0, w0, c, r;
        cin >> r0 >> w0 >> c >> r;
        if (r0 == 0 && w0 == 0 && c == 0 && r == 0) {
            return 0;
        }

        // C = (R_0 + X * R) / (W_0 + Y)
        // Y = (R_0 + X * R - C * W_0) / C
        for (int i = 0; true; ++i) {
            if (r0 + i * r - c * w0 >= 0) {
                cout << i << endl;
                break;
            }
        }
    }
}