#include <cstdio>
#include <algorithm>
#include <stack>
#include <queue>
#include <deque>
#include <vector>
#include <string>
#include <string.h>
#include <cstdlib>
#include <ctime>
#include <cmath>
#include <map>
#include <set>
#include <iostream>
#include <sstream>
#include <numeric>
#include <cctype>
#include <bitset>
#define fi first
#define se second
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define drep(i,n) for(int i = (n)-1; i >= 0; --i)
#define gep(i,g,j) for(int i = g.head[j]; i != -1; i = g.e[i].next)
#define each(it,c) for(__typeof((c).begin()) it=(c).begin();it!=(c).end();it++)
#define rng(a) a.begin(),a.end()
#define maxs(x,y) x = max(x,y)
#define mins(x,y) x = min(x,y)
#define pb push_back
#define sz(x) (int)(x).size()
#define pcnt __builtin_popcount
#define snuke srand((unsigned)clock()+(unsigned)time(NULL));
#define df(x) int x = in()
using namespace std;
typedef long long int ll;
typedef pair<int,int> P;
typedef vector<int> vi;
typedef vector<vi> vvi;
inline int in() { int x; scanf("%d",&x); return x;}
inline void priv(vi a) { rep(i,sz(a)) printf("%d%c",a[i],i==sz(a)-1?'\n':' ');}

const int MX = 100005, INF = 1001001001;
const ll LINF = 1e18;
const double eps = 1e-10;

int n;
double dp[105][105][105];

double dfs(vi a) {
  if (a[0] == 0 && a[1] == 0 && a[2] == 0) return 0;
  double& res = dp[a[0]][a[1]][a[2]];
  if (res != 0) return res;
  double p = 1./n;
  res = 1;
  rep(i,3) {
    if (a[i] == 0) continue;
    vi b = a;
    b[i]--;
    if (i != 2) b[i+1]++;
    res += a[i]*p*dfs(b);
  }
  res /= (a[0]+a[1]+a[2])*p;
  return res;
}

int main() {
  cin >> n;
  vi a(n);
  rep(i,n) cin >> a[i];
  vi ct(3);
  rep(i,n) {
    if (a[i] < 3) ct[a[i]]++;
  }
  double ans = dfs(ct);
  printf("%.10f\n", ans);
  return 0;
}