#include <iostream>
#include <fstream>
#include <cstdio>
#include <algorithm>
#include <deque>
#include <cmath>
#include <string>

using namespace std;

main()
{
	int n, t;
	ifstream cin("D.txt");
	while(cin>>n>>t && n+t)
	{
		double dp[2000+1];

		{
			string s; cin >> s;
			for(int i=0; i<n; i++) dp[i] = (s[i]=='R') ? 1.0 : 0.0;
		}

		for(int i=0; i<t; i++)
		{
			double dp1[2000+1] = {0.0};
			for(int j=0; j<n; j++)
			{
				for(int k=1; k<=6; k++)
				{
					dp1[(j+k)%n] += dp[j];
				}
			}

			for(int j=0; j<n; j++) dp[j] = dp1[j]/6.0;
		}
		double ans =  *max_element(dp, dp+n);
		printf("%.8f\n", ans);

	}
	return 0;
}