#pragma GCC optimize("O3")
#pragma GCC optimize(2)
#pragma GCC optimize(3)
#pragma GCC optimize("Ofast")
#pragma GCC optimize("inline")
#pragma GCC optimize("-fgcse")
#pragma GCC optimize("-fgcse-lm")
#pragma GCC optimize("-fipa-sra")
#pragma GCC optimize("-ftree-pre")
#pragma GCC optimize("-ftree-vrp")
#pragma GCC optimize("-fpeephole2")
#pragma GCC optimize("-ffast-math")
#pragma GCC optimize("-fsched-spec")
#pragma GCC optimize("unroll-loops")
#pragma GCC optimize("-falign-jumps")
#pragma GCC optimize("-falign-loops")	
#pragma GCC optimize("-falign-labels")
#pragma GCC optimize("-fdevirtualize")
#pragma GCC optimize("-fcaller-saves")
#pragma GCC optimize("-fcrossjumping") 
#pragma GCC optimize("-fthread-jumps")
#pragma GCC optimize("-funroll-loops")
#pragma GCC optimize("-freorder-blocks")
#pragma GCC optimize("-fschedule-insns")
#pragma GCC optimize("inline-functions")
#pragma GCC optimize("-ftree-tail-merge")
#pragma GCC optimize("-fschedule-insns2")
#pragma GCC optimize("-fstrict-aliasing")
#pragma GCC optimize("-falign-functions")
#pragma GCC optimize("-fcse-follow-jumps")
#pragma GCC optimize("-fsched-interblock")
#pragma GCC optimize("-fpartial-inlining")
#pragma GCC optimize("no-stack-protector")
#pragma GCC optimize("-freorder-functions")
#pragma GCC optimize("-findirect-inlining")
#pragma GCC optimize("-fhoist-adjacent-loads")
#pragma GCC optimize("-frerun-cse-after-loop")
#pragma GCC optimize("inline-small-functions")
#pragma GCC optimize("-finline-small-functions")
#pragma GCC optimize("-ftree-switch-conversion")
#pragma GCC optimize("-foptimize-sibling-calls")
#pragma GCC optimize("-fexpensive-optimizations")
#pragma GCC optimize("inline-functions-called-once")
#pragma GCC optimize("-fdelete-null-pointer-checks")
#pragma GCC optimize("Ofast")
//#pragma GCC target("avx,avx2,fma") 
//#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native") 
//#pragma GCC optimization ("unroll-loops")

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

long double dp[101][101][101];
bool check[101][101][101];
int n,t;

long double solve(int a,int b,int c)
{
    if(check[a][b][c])
    {
        return dp[a][b][c];
    }
    check[a][b][c] = true;
    if(a==0 && b==0 && c==0)
    {
        dp[a][b][c] = 0;
        return dp[a][b][c];
    }

    int d = n - (a+b+c);

    long double value = 1;
    if(a-1 >= 0)
    {
        value += ((a*solve(a-1,b+1,c))/n);
    }
    if(b-1 >= 0)
    {
        value += ((b*solve(a,b-1,c+1))/n);
    }
    if(c-1 >= 0)
    {
        value += ((c*solve(a,b,c-1))/n);
    }
    value*=n;
    value/=(n-d);
    dp[a][b][c] = value;
    return dp[a][b][c];
}

int main(void)
{
	cin.tie(0);
	ios::sync_with_stdio(false);

    cin >> n;

    int cnt[3];
    memset(cnt,0,sizeof(cnt));
    for(int i=0;i<n;i++)
    {
        cin >> t;
        if(t < 3)
        {
            cnt[t] += 1;
        }
    }

    memset(check,false,sizeof(check));

    cout << fixed;
    cout.precision(16);

    cout << solve(cnt[0],cnt[1],cnt[2]) << '\n';

	return 0;
}