CF312B.Archer

SmallR is an archer. SmallR is taking a match of archer with Zanoes. They try to shoot in the target in turns, and SmallR shoots first. The probability of shooting the target each time is for SmallR while for Zanoes. The one who shoots in the target first should be the winner.

Output the probability that SmallR will win the match.

小R是一名弓箭手。小R正与扎诺斯进行一场射箭比赛。他们轮流射击靶子，小R先射。小R每次射击命中靶子的概率为 ，而扎诺斯每次射击命中靶子的概率为 。首先命中靶子者获胜。

输出小R赢得该场比赛的概率。

A single line contains four integers .

一行包含四个整数 。

Print a single real number, the probability that SmallR will win the match.

The answer will be considered correct if the absolute or relative error doesn't exceed 10 - 6.

输出一个实数，表示 SmallR 获胜的概率。

若答案的绝对误差或相对误差不超过 $10^{-6}$，则视为正确。