#include <cstdio>
#include <vector>
#include <array>
#define repeat(i,n) for (int i = 0; (i) < (n); ++(i))
using namespace std;
template <typename T, typename X> auto vectors(T a, X x) { return vector<T>(x, a); }
template <typename T, typename X, typename Y, typename... Zs> auto vectors(T a, X x, Y y, Zs... zs) { auto cont = vectors(a, y, zs...); return vector<decltype(cont)>(x, cont); }
int main() {
    int n; scanf("%d", &n);
    vector<int> a(n); repeat (i,n) scanf("%d", &a[i]);
    array<int,3> cnt = {}; repeat (i,n) if (a[i] < 3) cnt[a[i]] += 1;
    array<int,3> l; repeat (i,3) l[i] = (i ? l[i-1] : 0) + cnt[i];
    vector<vector<vector<double> > > dp = vectors(double(), l[0]+1, l[1]+1, l[2]+1);
    repeat (i,l[0]+1) {
        repeat (j,l[1]+1-i) {
            repeat (k,l[2]+1-i-j) {
                if (i == 0 and j == 0 and k == 0) continue;
                double p = (i + j + k) /(double) n;
                double qi = i /(double) (i + j + k);
                double qj = j /(double) (i + j + k);
                double qk = k /(double) (i + j + k);
                dp[i][j][k] = 1/p;
                if (i-1 >= 0 and j+1 < l[1]+1) dp[i][j][k] += qi * dp[i-1][j+1][k];
                if (j-1 >= 0 and k+1 < l[2]+1) dp[i][j][k] += qj * dp[i][j-1][k+1];
                if (k-1 >= 0                 ) dp[i][j][k] += qk * dp[i][j][k-1];
            }
        }
    }
    printf("%.12f\n", dp[cnt[0]][cnt[1]][cnt[2]]);
    return 0;
}