#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;

const int INF = 1e9 + 7;

int main()
{
	ios::sync_with_stdio(false);
	cin.tie(0);
	
	int c, t, r;
	cin >> c >> t >> r;
	double p;
	cin >> p;
	
	vector<double> dp(c + 1);
	dp[0] = 0;
	FOR (i, 1, c + 1)
	{
		dp[i] = (dp[i - 1] + 1 + r * p) / (1 - p);
	}
	vector<double> dp2(c + 1, INF);
	dp2[0] = 0;
	FOR (i, 1, c + 1)
	{
		FOR (j, 0, i)
		{
			dp2[i] = min(dp2[i], dp2[j] + dp[i - j] + t);
		}
	}
	cout << fixed << setprecision(10) << dp2[c] << '\n';
	return 0;
}