using System;

class EstimatePI
{
	static double GetPI(int num_terms)
	{
		double denom;
		double fourth_pi = 0.0;
		int sign = 1;

		//PI/4 = 1 - 1/3 + 1/5 - 1/7 + 1/9 - 1/11 + ....
		for(int i=1; i <= num_terms; i++)
		{
			denom = 2*i-1;

			fourth_pi += sign*(1.0/denom);

			sign = -sign;
		}

		return 4.0*fourth_pi;
	}

	public static void Main()
	{
		Console.WriteLine("Estimate of PI with {0} terms is {1}\n", 100, GetPI(100));
		Console.WriteLine("Estimate of PI with {0} terms is {1}\n", 1000, GetPI(1000));
		Console.WriteLine("Estimate of PI with {0} terms is {1}\n", 1000000, GetPI(1000000));

		Console.WriteLine("Math.PI = {0}. ", Math.PI);


	}


}