#include<bits/stdc++.h>
#define _ ios_base::sync_with_stdio(0);cin.tie(0);
#define REP(i,n) for(int i=0;i<(int)(n);i++)

using namespace std;

void solve(int r0,int w0,int c,int r){
  for(int i=0;;r0+=r,i++){
    if(r0>=w0*c){
      cout<<i<<endl;
      return;
    }
  }
}

int main(){ _;
  int r0,w0,c,r;
  while(cin>>r0>>w0>>c>>r,(r0|w0|c|r)!=0){
    solve(r0,w0,c,r);
  }
}