求PI，为什么结果不对，还是逻辑上是错的？

double n=3;
double PI;
double s=1/n;
double sum=1/3;
do
{

           sum = sum + s;
            n = n + 2;
            PI = 4 *(1-sum);
        }
        while (s < 0.000001);
        Console.WriteLine(PI);*/

先理解算法
 double n=3;
            double PI;
            double s=1/n;
            int i = 1;
            double sum=1;
            do
            {
                i = i * -1;
                s = 1 / n; 
                sum = sum + s*i;
                n = n + 2;
            }
            while (s > 0.000001);
            PI = 4 * sum;
            Console.WriteLine(PI);