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