#include <bits/stdc++.h>
using namespace std;
int main(void){
    // Your code here!
    int a,b,c,d;
    while(cin>>a>>b>>c>>d,d){
        int ans=0;
        while(1){
            if((ans*d+a)>=b*c){
                cout << max(ans,0)<<endl;
                break;
            }
            ans++;
        }
    }
}