using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace ConsoleApplication2
{
    class Program
    {
        static void Main(string[] args)
        {
            while (true)
            {
                //π/4  = 1 -1/3  +1/5  -1/7  +1/9  -1/11  +……，精确至少4位小数.
                Console.WriteLine("请输入运算次数进行下一次计算...");
                decimal a = decimal.Parse(Console.ReadLine());
                Console.WriteLine("输入的次数: {0}", a);
                decimal result = 0;
                bool add = true;
                for (decimal i = 1; i < a; i += 2)
                {
                    //result += 1d / (2d * i - 1d) * Math.Pow(-1d, i - 1d);
                    if (add)
                    {
                        result += 1m / i;
                        add = false;
                    }
                    else
                    {
                        result -= 1m / i;
                        add = true;
                    }
                    //Console.WriteLine(result * 4d);
                }
                Console.WriteLine("PI的结果为: {0}", (result * 4m));
                Console.WriteLine("PI的结果为: {0}", (result * 4m).ToString("#0.0000"));
            }
        }
    }
}