AOJ2745.cpp

#include <bits/stdc++.h>
 
using namespace std;
 
int main() {
  int R0, W0, C, R;
  while (cin >> R0 >> W0 >> C >> R, C) {
    int rest = max(W0 * C - R0, 0);
    cout << (rest + R - 1) / R << endl;
  }
  return 0;
}