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

#define SZ(a) (int)a.size()
#define ALL(a) a.begin(), a.end()
#define FOR(i, a, b) for (int i = (a); i<(b); ++i)
#define RFOR(i, b, a) for (int i = (b)-1; i>=(a); --i)
#define MP make_pair
#define PB push_back
#define F first
#define S second
#define FILL(a, b) memset(a, b, sizeof(a))

typedef long long LL;
typedef pair<int, int> PII;
typedef vector<int> VI;

typedef long double db; 

const int N = 1 << 12;
db f[N], pw[N], npw[N];


db calc(db p, int n){
	db res = (1 - p) * (1 - npw[n]) / p  - npw[n + 1] * n;
	return res / p;
}

db dp[N];


int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	int n, save, rest;
	cin >> n >> save >> rest;
	
	db p;
	cin >> p;
	
	pw[0] = npw[0] = 1;
	FOR(i, 1, N){
		pw[i] = pw[i - 1] * p;
		npw[i] = npw[i - 1] * (1 - p);
	}	
	FOR(len, 1, N){
		db l = npw[len];
		db r = npw[len] * len; 
		r += (rest + 1) * (1 - npw[len]);
		r += p * calc(p, len - 1);
		
		
		f[len] = r / l;
	}
	
	FOR(i, 1, n + 1){
		dp[i] = f[i];
		FOR(j, 0, i)
			dp[i] = min(dp[i], dp[j] + f[i - j]);
		dp[i] += save;
	}
	cout << fixed << setprecision(10) << dp[n] << endl;
	
	


	
	
	
	return 0;
}