#include <sstream>
#include <string>
#include <vector>
#include <map>
#include <algorithm>
#include <iostream>
#include <utility>
#include <set>
#include <cctype>
#include <queue>
#include <stack>
#include <cstdio>
#include <cstdlib>
#include <cmath>

#define INF 1000000000

using namespace std;
typedef long long ll;
int N;
double memo[110][110][110];

double dfs(int cnt0, int cnt1, int cnt2) {
    if (memo[cnt0][cnt1][cnt2] >= 0) return memo[cnt0][cnt1][cnt2];
    if (cnt0 == 0 && cnt1 == 0 && cnt2 == 0) return 0;
    int sum = cnt0 + cnt1 + cnt2;
    double ret = N * 1.0 / sum;
    if (cnt0 > 0) {
        ret += (1.0*cnt0) / sum * (dfs(cnt0-1, cnt1+1, cnt2));
    }
    if (cnt1 > 0) {
        ret += (1.0*cnt1) / sum * (dfs(cnt0, cnt1-1, cnt2+1));
    }
    if (cnt2 > 0) {
        ret += (1.0*cnt2) / sum * (dfs(cnt0, cnt1, cnt2-1));
    }
    return memo[cnt0][cnt1][cnt2] = ret;
}

int main(void) {
    int cnt0 = 0, cnt1 = 0, cnt2 = 0;
    cin >> N;
    for (int i = 0; i <= N; i++) {
        for (int j = 0; j <= N; j++) {
            for (int k = 0; k <= N; k++) memo[i][j][k] = -1;
        }
    }
    for (int i = 0; i < N; i++) {
        int a;
        cin >> a;
        if (a == 0) cnt0++;
        if (a == 1) cnt1++;
        if (a == 2) cnt2++;
    }
    printf("%.10lf\n", dfs(cnt0, cnt1, cnt2));
    return 0;
}