/*

Author: Haze

2024/4/24

*/

#include <bits/stdc++.h>

#define IOS ios::sync_with_stdio(false), cin.tie(nullptr);
using namespace std;
using ll = long long;
#define int ll

#define debugs(x) cerr<<#x<<" "<<x<<endl;
//const int mod = 1000000000 + 7;
//const int itinf = 1000000999;
//const ll llinf = 2e18;
//const int N = 500099;

void solve() {
    int c, t, r;
    long double q;
    cin >> c >> t >> r >> q;
    vector<long double> f1(c + 1);
    vector<long double> f2(c + 1, -1);
    for (int i = 1; i <= c; i++) {
        f1[i] = (f1[i - 1] + 1.0) / (1.0 - q) + r * q / (1.0 - q);
    }
    f2[0] = 0;
//    for (int i = 1; i <= c; i++)
//        cerr << fixed << setprecision(12) << f1[i] << endl;
    for (int i = 1; i <= c; i++) {
        for (int j = 0; j < i; j++) {
            if (f2[i] == -1) f2[i] = f2[j] + f1[i - j];
            else f2[i] = min(f2[i], f2[j] + f1[i - j]);
//            debugs(i);
//            debugs(j);
//            debugs(f2[i]);
//            debugs(f2[j])
//            debugs(f1[i - j]);
//            f2[i] = min(f2[i], f2[j] + f1[i - j]);
        }
        f2[i] += t;
    }
//    for (int i = 1; i <= c; i++)
//        cerr << fixed << setprecision(12) << f2[i] << endl;
    cout << fixed << setprecision(12) << f2[c] << endl;

}

signed main() {
//    IOS
    int T = 1;
    while (T--) {
        solve();
    }
    return 0;
}