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

typedef long long ll;
typedef long double ld;
typedef pair<ll, ll> ii;
typedef vector<ll> vi;
typedef vector<vi> vvi;
typedef vector<vvi> vvvi;
typedef vector<vvvi> vvvvi;
typedef vector<ii> vii;

#define x first
#define y second
#define pb push_back
#define eb emplace_back
#define all(v) (v).begin(), (v).end()

void run() {
	ll c;
	ld t, r;
	cin >> c >> t >> r;
	ld p;
	cin >> p;
	vector<ld> A(c+1, 0);
	for (ll i = 1; i <= c; i++) {
		A[i] = (A[i-1] + 1 + p*r)/(1-p);
	}
	vector<ld> B(c+1, 0);
	for (ll i = 1; i <= c; i++) {
		B[i] = A[i]+t;
		for (ll j = 1; j <= i; j++) {
			B[i] = min(B[i], B[i-j] + A[j] + t);
		}
	}
	cout << B[c] << endl;
}

signed main() {
    ios_base::sync_with_stdio(false); cin.tie(0);
	cout << fixed << setprecision(20);
    run();
    return 0;
}