double a = 0.0;//最终Π的结果 double类型
            int n;
            for (n = 1; n <= 2000; n++)
            {
                if (n % 2 == 1)
                {
                    a += 1.0 / (2 * n - 1);
                }
                else
                {
                    a -= 1.0 / (2 * n - 1);
                }


            }
            a *= 4;
            Console.WriteLine(a);
            Console.ReadKey();