#include<iostream>
#include<algorithm>

using namespace std;

int main(){
 int r0,w0,c,r,i;
 while(cin >> r0 >> w0 >> c >> r && r0){
  i=0;
  while((r0+i*r)/w0 < c)i++;
  cout << i << endl;
 }
 return 0;
}
