#include <cstdio>
using namespace std;

#define N 2002
int n,t;
double dice[2][N];
char s[N];

int main() {
  while(scanf("%d%d",&n,&t)==2&&(n|t)) {
    gets(s);
    gets(s);

    for (int i=0;i<n;++i) dice[0][i]=0;
    dice[0][0]=1;
    for (int tt=0;tt<t;++tt) {
      for (int i=0;i<n;++i) dice[tt+1&1][i]=0;
      for (int i=0;i<n;++i)
	if (dice[tt&1][i]) {
	  double v=dice[tt&1][i]/6;
	  for (int j=1;j<=6;++j)
	    dice[tt+1&1][(i+j)%n]+=v;
	}
    }
    double ans=0;
    for (int i=0;i<n;++i) {
      double cnt=0;
      for (int j=0;j<n;++j)
	if (s[(i+j)%n]=='R')
	  cnt+=dice[t&1][j];
      ans>?=cnt;
    }
    printf("%.8lf\n",ans);
  }
}