#include <bits/stdc++.h>
using namespace std;

int main(){
    while(1){
        int SR,SW,C,R;
        cin>>SR>>SW>>C>>R;
        if(SR == 0 && SW == 0 && C == 0 && R == 0) break;
        int cur = SR,ans = 0;

        while(cur/SW < C){
            cur += R;
            ans++;
        }

        cout<<ans<<endl;   
    }

    return 0;
}
