#include <bits/stdc++.h>
#define REP(i, n) for(int i = 0; i < (int) n; ++i)
#define FOR(i, j, k) for(int i = (j); i <= (k); ++i)
#define FORD(i, j, k) for(int i = (j); i >= (k); --i)

using namespace std;

typedef long double ld;

int w, b;
int n;
set<string> vars;
map<string, int> vars_map;
string vars_arr[321];


bool is_zero(ld x){
	return abs(x) < 0.000000001;
}


ld a[321][321], ans[321];
bool gauss(){
	REP(i, n){
		if(is_zero(a[i][i])){
			FOR(j, i+1, n-1)if(!is_zero(a[j][i])){
				FOR(k,i,n) swap(a[i][k],a[j][k]);
				break;
			}
		if(is_zero(a[i][i])) return false;
		}
		ld inv = 1.0/a[i][i];
		FOR(j, i, n)a[i][j]*=inv;
		FOR(j, i+1, n-1) FORD(k, n, i){
			a[j][k]-=a[i][k]*a[j][i];
		}
	}
	FORD(i, n-1, 0) REP(j, i){
		a[j][n]-=a[i][n]*a[j][i];
		FOR(k, 0, i)a[j][k]-=a[i][k]*a[j][i];
	}
	REP(i, n) ans[i]=a[i][n];
	return true;
}




string shorten(string x, char nw){
	x.push_back(nw);
	while(vars.find(x) == vars.end()){
		REP(i, (int)x.size() -1){
			x[i] = x[i+1];
		}
		x.pop_back();
	}
	return x;
}


void test(){
	n=2;
	a[0][0] = 1; a[0][1] = 0; a[0][2] = 1;
	a[1][0] = 1; a[1][1] = 1; a[1][2] = 0.7;
	assert(gauss());
	cerr << ans[0] << " " << ans[1] << endl;
}

int main(){
	cin >> w >> b;
	string x;
	REP(i, w){
		cin >> x;
		while(1){
			vars.insert(x);
			if(x.empty()) break;
			x.pop_back();
		}
	}

	n=0;
	for(auto && s : vars){
		vars_arr[n] = s;
		vars_map[s] = n++;
	}
	assert(n < 311);

	REP(i, n+1) REP(j, n+1) a[i][j]=0;
	
	REP(i, n){
		a[i][i] += 1;
		if(vars_arr[i].length() == b){
			a[i][n]=0;
		} else{
			a[i][n]=1;
			a[i][vars_map[shorten(vars_arr[i], 'H')]]-=0.5;
			a[i][vars_map[shorten(vars_arr[i], 'T')]]-=0.5;
		}
	}	

	/*REP(i, n){
		cout << vars_arr[i] << endl;
	}

	REP(i, n){
		REP(j, n+1) cout << a[i][j] << " ";
		cout << endl;
	}*/


	

	assert(gauss());

	//REP(i, n) cout << ans[i] << endl;

	string empty="";
	cout << fixed << setprecision(6) << (ans[vars_map[empty]]) << endl;
	

	return 0;
}