#include <iostream>

using namespace std;

int main() {
  while (true) {
    int r, w, c, r_;
    cin >> r >> w >> c >> r_;
    if (r == 0) break;
    
    int x = 0, y = 0;
    while (r + x * r_ < c * w + c * y) {
      x++;
    }
    
    cout << x << endl;
  }
}
