说明：  利用函数和递归做一个数列的加减运算，数列的加减运算有一定的规律，可利用函数与计算出结果
(The constant  can be calculated by the infinite series:  = 4- 4/3+ 4/5- 4/7+ 4/9- 4/11+.... Write a C program that uses a do/while loop to calculate  using the series. The program should ask the user how many terms in the series should be used. Thus if the user enters ‘3’, then the program should calculate  as being 4- 4/3+ 4/5.)