#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
#define int ll

int n ; 
long double dp[101][101][101] ; 
long double solve(int x , int y , int z){
    if (x == n || y == n || z == n){
        return 0.0 ; 
    }
    if( x + y == n){
        return 3.0 * y ; 
    }
    if(dp[x][y][z] != 0) return dp[x][y][z] ; 
    long double sum = 1.5 ; 
    sum += 0.5 * solve(x+1 , y , z - 1) ;
    
    int w = x ; int l = y + 1 ; 
    if (l > w) swap(l , w) ; 

    sum += 0.5 * solve(w , l , z-1) ; 

    return dp[x][y][z] =  sum ; 
}

signed main()
{
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cin >> n ; 
    string s ; cin >> s ; 
    vector<int> v(3) ; 
    for(int i = 0 ; i < n ; i++){
        if(s[i] == 'r') v[0]++; 
        if(s[i] == 'g') v[1]++;
        if(s[i] == 'b') v[2]++; 
    }
    sort(v.begin() , v.end()) ; 

    cout << fixed << setprecision(8) << solve(v[2] , v[1] , v[0]) << endl ;

}