import sys; input = sys.stdin.buffer.readline
sys.setrecursionlimit(10**7)
from collections import defaultdict
con = 10 ** 9 + 7; INF = float("inf")

def getlist():
	return list(map(int, input().split()))

#処理内容
def main():
	vl, vr = getlist()
	d = int(input())
	w = int(input())
	ans = w * d / (vl + vr)
	print(ans)
	




if __name__ == '__main__':
	main()