#include<bits/stdc++.h>
using namespace std;
#define rep(i,a,b) for(int i=a;i<b;i++)



int N;
int A[101];
//-----------------------------------------------------------------
double memo[101][101][101];
double rec(int a, int b, int c) {
	if (0 <= memo[a][b][c]) return memo[a][b][c];
	int sm = a + b + c;
	if (sm == 0) return 0;

	double ret = ((double)N / (double)sm);
	
	if (0 < a) ret += rec(a - 1, b + 1, c) * ((double)a / (double)sm);
	if (0 < b) ret += rec(a, b - 1, c + 1) * ((double)b / (double)sm);
	if (0 < c) ret += rec(a, b, c - 1) * ((double)c / (double)sm);

	return memo[a][b][c] = ret;
}

int main() {
	cin >> N;
	rep(i, 0, N) cin >> A[i];

	int a = 0, b = 0, c = 0;
	rep(i, 0, N) {
		if (A[i] == 0)
			a++;
		else if (A[i] == 1)
			b++;
		else if (A[i] == 2)
			c++;
	}

	rep(i, 0, N + 1) rep(j, 0, N + 1) rep(k, 0, N + 1) memo[i][j][k] = -1;
	printf("%.10f\n", rec(a, b, c));
}