import functools

c, s, r = [int(x) for x in input().split()]
p = float(input())

@functools.cache
def expected_time_no_saving(chars, r, p):
    if chars == 0:
        return 0

    return (expected_time_no_saving(chars - 1, r, p) + 1 + p * r) / (1 - p)

# ends on save
@functools.cache
def expected_total_time(chars, s, r, p):
    if chars == 0:
        return 0

    min_time = None

    for last_save_after in range(0, chars):
        this_time = (
            expected_total_time(last_save_after, s, r, p) +
            expected_time_no_saving(chars - last_save_after, r, p) + s
        )

        if min_time is None or this_time < min_time:
            min_time = this_time

    return min_time

print(expected_total_time(c, s, r, p))