Calculating Pi from random numbers

PiDay.cs

using System;

namespace PiDay
{
	// The probability of two random numbers to be coprime is P = 6 / Pi^2
	// So let's generate a bunch of random numbers. Figure out how often they're coprime.
	// Use that percentage as our probabilty and solve for Pi.
	// Pi = sqrt(6 / P)

	// For more info, watch the video where I stole the idea: https://youtu.be/RZBhSi_PwHU

	class Program
	{
		static void Main(string[] args)
		{
			int pairsToTest = (args.Length == 1) ? int.Parse(args[0]) : 10000000;
			int coprimeCount = 0;

			Random r = new Random();

			for (int i = 0; i < pairsToTest; i++)
			{
				int r1 = r.Next();
				int r2 = r.Next();

				if (Coprime(r1, r2))
				{
					coprimeCount++;
				}
			}

			double percentCoprime = coprimeCount / (double)pairsToTest;
			double calculatedPi = Math.Sqrt(6.0 / percentCoprime);

			Console.WriteLine(string.Format("We calculated Pi to be: {0}", calculatedPi));
			return;
		}

		public static int GetGCD(int a, int b)
		{
			while (b != 0)
			{
				var t = a % b;
				a = b;
				b = t;
			}

			return a;
		}

		public static bool Coprime(int a, int b)
		{
			if (((a | b) & 1) != 1)
				return false;

			return GetGCD(a, b) == 1;
		}
	}
}