#include<bits/stdc++.h>

#define lp(i,n) for(int i=0;i<n;i++)
#define lpin(a,i,n) for(int i=0;i<n;i++){cin>>a[i];}
using namespace std;

int main(){
  while(1){
    int r,w,c,g;
    cin>>r>>w>>c>>g;
    if(r==0&&w==0&&c==0&&g==0) break;
    int x=0;
    while(1){
      double a;
      a=r/w;
      if(a>=c) break;
      else{
	r+=g;
	x++;
      }
    }
    cout<<x<<endl;
  }
  return 0;
}

