Smeech

Problem Description
Professor Octastichs has invented a new programming language, Smeech. An expression in Smeech may be a positive or negative integer, or may be of the form (p e1 e2) where p is a real number between 0 and 1 (inclusive) and e1 and e2 are Smeech expressions. The value represented by a Smeech expression is as follows:
1) An integer represents itself
2) With probability p, (p e1 e2) represents x+y where x is the value of e1 and y is the value of e2; otherwise it represents x-y.
Given a Smeech expression, what is its expected value?

Input
Input consists of several Smeech expressions, one per line, followed by a line containing ().

Output
For each expression, output its expected value to two decimal places.

Sample Input
7
(.5 3 9)
()

Sample Output
7.00
3.00

http://poj.org/problem?id=3645

问题描述
Octastichs教授发明了一种新的编程语言，Smeech。在Smeech的表达式可以是正的或负的整数，也可以是形式（P E1 E2）其中p是0和1之间的实数（含），E1和E2是浓烟的表达。通过浓烟表达值如下：
1）整数代表自己
2）的概率为p（p E1 E2）表示x + y，x是y的值是E1和E2的值；否则，它代表x-y.
给定一个浓烟的表达，其期望值是什么？
输入
输入包含多个浓烟的表情，每行一个，包含了（）。
输出
对于每个表达式，输出其期望值到小数点的两位。
样本输入
7
(.5 3 9)
()
示例输出
7.00
3.00