#include <bits/stdc++.h>
using namespace std;

int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);

    int N; cin >> N;
    int zero = 0,one = 0,two = 0;
    for(int i=0; i<N; i++){
        int a; cin >> a;
        if(a == 0) zero++;
        if(a == 1) one++;
        if(a == 2) two++;
    }

    long double answer = 0;
    vector dp(N+1,vector(N+1,vector<long double>(N+1)));
    dp.at(zero).at(one).at(two) = 1;
    for(int i=N; i>=0; i--) for(int k=N; k>=0; k--) for(int l=N; l>=0; l--){
        if(i+k+l == 0) continue;
        long double d = dp.at(i).at(k).at(l);
        int left = N-i-k-l;
        long double mul = N/(N-left+0.0),div = 1.0/(i+k+l);
        answer += d*mul;
        if(i) dp.at(i-1).at(k+1).at(l) += i*div;
        if(k) dp.at(i).at(k-1).at(l+1) += k*div*d;
        if(l) dp.at(i).at(k).at(l-1) += l*div*d;
    }

    cout << fixed << setprecision(20) << answer << endl;
}