#include <iostream>
using namespace std;

int main()
{
  int R0, W0, C, R;
  while(1){
    cin >> R0 >> W0 >> C >> R;
    if(R0 == 0 && W0 == 0 && C == 0 && R == 0) break;
    int ans = 0;
    while((ans++)*R+R0 < W0*C){}
    cout << ans-1 << endl;
  }
}