#include <cmath>
#include <cstring>
#include <iomanip>
#include <iostream>

using namespace std;

typedef long double ld;

int main() {
    int c;
    long long t, r;
    ld p;
    cin >> c >> t >> r >> p;

    // time_to_type, dp = [0] * (c + 1), [[0] * (c + 1) for _ in range(c + 1)]
    ld time_to_type[c + 1], dp[c + 1][c + 1];
    time_to_type[0] = 0;
    for (int i = 0; i <= c; i++) {
        for (int j = 0; j <= c; j++)
            dp[i][j] = 0;
    }

    // time_to_type[x] is the expected time to type x characters without saving at any time.
    // Derived from: time_to_type[x] = dp[0][x] + 1-p + p * (1 + r + time_to_type[x])
    for (int i = 1; i <= c; i++) {
        time_to_type[i] = (time_to_type[i - 1] + 1 + p * r) / (1 - p);
    }

    // dp[x][y] is the time required to type all characters between indices x (inclusive) and y (exclusive).
    for (int l = 1; l <= c; l++) {
        for (int s = 0; s <= c - l; s++) {
            int e = s + l;
            dp[s][e] = min(time_to_type[e - s], 1e18l);
            for (int k = s + 1; k < e; k++)
                dp[s][e] = min(dp[s][e], dp[s][k] + t + dp[k][e]);
        }
    }

    cout << setprecision(20) << dp[0][c] + (ld) t << endl;
}