#include <iostream>
using namespace std;

int main(void){
  while(true){
    int r0, w, c, r;
    cin >> r0 >> w >> c >> r;
    if(r0 == 0 && w == 0 && c == 0 && r == 0){ break; }

    int x = (c * w - r0 + (r - 1)) / r;
    cout << (x > 0 ? x : 0) << endl;
  }

  return 0;
}