关于C#求π的值，求高手分析下程序

namespace ConsoleApplication1
{
class Program
{
static void Main(string[] args)
{
float p = 0;int n = 2000;
for (n = 1; n <= 2000; n++)
{
p=p+ 1/(4 * p - 3) - 1/(4 * p - 1);
}
p = 4*p;
Console.WriteLine("p={0}", p);
}
}
}

最后的运行结果显示是负数。。。求问一下程序哪里出错了
题目是
π/4=1-1/3+1/5-1/7…+1/(4n-3)-1/(4n-1) (n=2000)

class Program
{
static void Main(string[] args)
{
float p = 0;int n = 2000;
for (n = 1; n <= 2000; n++)
{
if(n % 2 == 0)
{

p = p - 1/(4 * n - 1);
}
else
{
p = p + 1/(4 * n - 3);

}

}
p = 4*p;
Console.WriteLine("p={0}", p);
}
}

double p = 0;

我看看

namespace ConsoleApplication1 { class Program { static void Main(string[] args) { float p = 0; int n ; for (n = 1; n < =2000; n++) { p=p+ 1/(4 * n - 3) - 1/(4 * n - 1); }; p = 4*p; Console.WriteLine("p={0}", p); } } }