import java.io.*;
import java.util.*;

public class Maarten {
    public static void main(String[] args) throws IOException {
        try (BufferedReader reader = new BufferedReader(new InputStreamReader(System.in))) {
            new Maarten().run(new Scanner(reader));
        }
    }

    public void run(Scanner sc) throws IOException {
        int c = sc.nextInt();
        long t = sc.nextLong(), r = sc.nextLong();
        double p = sc.nextDouble();

        double[] time_to_type = new double[c + 1], dp = new double[c + 1];

        for (int i = 1; i <= c; i++) {
            time_to_type[i] = (time_to_type[i - 1] + 1 + p * r) / (1 - p);
        }

        for (int l = 1; l <= c; l++) {
            dp[l] = time_to_type[l];
            for (int k = 0; k < l; k++) {
                dp[l] = Math.min(dp[l], dp[k] + t + time_to_type[l - k]);
            }
        }

        System.out.println(dp[c] + t);
    }
}