#!/usr/bin/env python3
c, t, r = map(int, input().split())
p = float(input())

# Time to type i chars after a save
# chars[i] = chars[i-1] + 1 + p * (r + chars[i])
chars = [0] * (c + 1)
for i in range(1, c + 1):
    chars[i] = (chars[i - 1] + 1 + p * r) / (1 - p)

# save[i]: optimal cost to save after char i
save = [0] * (c + 1)

for i in range(1, c + 1):
    # Last save was after char j.
    for j in range(i):
        cost = save[j] + chars[i - j] + t
        if save[i] == 0 or cost < save[i]:
            save[i] = cost

print(save[c])