using System;

namespace loto_probability
{
    class Program
    {
        static void Main(string[] args)
        {
            int totalLottoBalls = Convert.ToInt32(Console.ReadLine());
            int ballsExtracted = Convert.ToInt32(Console.ReadLine());
            string category = Console.ReadLine();
            switch (category)
            {
                case "II":
                    long denominator = Combinations(ballsExtracted,(ballsExtracted-1)) * Combinations(totalLottoBalls-ballsExtracted, (ballsExtracted-(ballsExtracted - 1)));
                    long nominator= Combinations(totalLottoBalls, ballsExtracted);
                    Console.Write(Probability(denominator, nominator));
                    break;
                case "III":
                    long denominator2 = Combinations(ballsExtracted, ballsExtracted - 2) * Combinations(totalLottoBalls - ballsExtracted, (ballsExtracted - (ballsExtracted - 2)));
                    long nominator2 = Combinations(totalLottoBalls, ballsExtracted);
                    Console.Write(Probability(denominator2, nominator2));
                    break;
                default:
                    Console.Write(Probability(1, Combinations(totalLottoBalls, ballsExtracted)));
                    break;

            }


        }
        static int Factorial(long number)
        {
            int factorial = 1;
            for(int i = 1; i <= number; i++)
            {
                factorial *= i;
            }
            return factorial;
        }
        static long Combinations(int n, int k)
        {
            long combinationsResult = Factorial(n) / (Factorial(k) * Factorial(n - k));
            return combinationsResult;
        }

        static double Probability(long goodCases,long totalCases)
        {

           double result=goodCases / totalCases;
            return Math.Round(result, 10);
        }


    }
}