# Enter your code here. Read input from STDIN. Print output to STDOUT

import math

numTic = float(input())
numStd = float(input())
mu = numStd * float(input())
sig = math.sqrt(numStd) * float(input())

print(round(0.5*(1+math.erf((numTic - mu)/(sig * math.sqrt(2)))),4))
