LeinitzPI

static double CalcPI()
        {
            double pi = 1;
            for (int i = 3, cnt = 0; i < 100000; i += 2, cnt++)
            {
                if (cnt % 2 == 0)
                {
                    pi -= (double)1 / i;
                }
                else
                {
                    pi += (double)1 / i;
                }
            }
            pi *= 4;
            return pi;
        }