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()