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

using ld = long double;
int N;
int A[101];
ld dp[101][101][101];
ld f(int c1, int c2, int c3) {
    if (c1 == c2 && c2 == c3 && c1 == 0) return 0.0;
    if (dp[c1][c2][c3] >= 0) return dp[c1][c2][c3];
    int c0 = N - c1 - c2 - c3;
    ld l = 1, r = 0;
    if (c1) r += (f(c1 - 1, c2, c3) + 1) / N * c1;
    if (c2) r += (f(c1 + 1, c2 - 1, c3) + 1) / N * c2;
    if (c3) r += (f(c1, c2 + 1, c3 - 1) + 1) / N * c3;
    if (c0) {
        r += ld(c0) / N;
        l -= ld(c0) / N;
    }
    return r / l;
}
int main() {
    cin >> N;
    for (int i = 0; i < N; i++) cin >> A[i];
    for (int i = 0; i <= N; i++)
        for (int j = 0; j <= N; j++)
            for (int k = 0; k <= N; k++) dp[i][j][k] = -1;
    int c1 = 0, c2 = 0, c3 = 0;
    for (int i = 0; i < N; i++) {
        c1 += A[i] == 2;
        c2 += A[i] == 1;
        c3 += A[i] == 0;
    }
    cout << fixed << setprecision(16) << f(c1, c2, c3) << "\n";
}