#include <bits/stdc++.h>
using namespace std;
int main(){
    cout.precision(2);
    cout << fixed;
    double l,p,a,n;
    cin >> l >> p >> a >> n;
    double so = double(l)*double(p);
    double zapremina = l;
    double p1 = so / zapremina;
    while(n--){
        so -= p1*a;
        p1 = so/zapremina;
    }
    cout << p1 << endl;
}