#!/usr/bin/env python3
import math
c,t,r = [int(i) for i in input().split()]
p = float(input())

DP = [0]
for x in range(1,c+1):
    DP.append(min([t + DP[x-i] + (r+1/p)*((1-p)**-i - 1) for i in range(1,min(x+1, int(-math.log(10**15,1-p))))]))
print(DP[c])