vl,vr = [int(i) for i in input().split()]
d = int(input())
w = int(input())

dn=d
d_sum=0
n=1
while(dn>0.00000001):
    v=vl if n%2==0 else vr
    t=dn/(w+v)
    dn=dn-(vl+vr)*t
    d_sum+=w*t

print(d_sum)