'''
Created on Mar 16, 2017

@author: Madalina
'''
string = input()
string = string.split()
t1 = int(string[0])
k1 = int(string[1])
t2 = int(string[2])
k2 = int(string[3])

t3 = t1 * t2 / (t1 + t2)

s = t1 + t2
k = k1 * t2 + k2 * t1
print((t3 * s )/ k)