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)

t=d/(vl+vr)
print(w*t)