Vl = 0
Vr = 0
d = 0
w = 0

def scan():
    l=map(int,raw_input().split())
    global Vl
    global Vr
    global d 
    global w 
    Vl = l[0]
    Vr = l[1]
    d = input()
    w = input()

scan()
time = float(d) / (Vl + Vr)
print w * time