s_data = input()
s_int = s_data.split(" ")

v_first = int(s_int[0])
v_sec = int(s_int[1])

d_int = int(input())
w_int = int(input())

v_com = v_first + v_sec
t_int = d_int/v_com

result = w_int*t_int

if int(w_int*t_int)%v_com == 0:
	print(int(result))
else:
	result = int(result*10000000)/10000000
	print(result)