#include <algorithm>
#include <cassert>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <iostream>
#include <numeric>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <functional>
#include <iomanip>
using namespace std;
using ll = long long;

class range {private: struct I{int x;int operator*(){return x;}bool operator!=(I& lhs){return x<lhs.x;}void operator++(){++x;}};I i,n;
public:range(int n_):i({0}),n({n_}){}range(int i_,int n_):i({i_}),n({n_}){}I& begin(){return i;}I& end(){return n;}};

template<typename T, typename U> ostream& operator<<(ostream& os, const pair<T, U>& p){ return os << "{" << p.first << ", " << p.second << "}"; }
template<typename T> ostream& operator<<(ostream& os, const vector<T>& obj) { os << "{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }
template<typename T> ostream& operator<<(ostream& os, const set<T>& obj) { os << "set{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }
template<typename T, typename U> ostream& operator<<(ostream& os, const map<T, U>& obj) { os << "map{"; for (const auto& e : obj) os << e << ", "; return os << "}"; }

#ifdef ONLINE_JUDGE
#define dump(expr) ;
#else
#define dump(expr) { cerr << "\033[33m#L" << __LINE__ << ": " << expr << "\033[39m" << endl; }
#endif

namespace solver {

int n;
vector<int> vs;

void read() {
  cin >> n;
  vs.resize(n);
  for (int i : range(n)) cin >> vs[i];
}

double dp[101][101][101];

double run() {
  dp[0][0][0] = 0;
  for (int zer : range(n + 1))
  for (int one : range(n + 1))
  for (int two : range(n + 1)) {
    int comp = n - (zer + one + two);
    if (comp < 0) continue;
    if (comp == n) continue;

    double d = 1.0;
    if (zer > 0) d += dp[zer - 1][one + 1][two] * zer / n;
    if (one > 0) d += dp[zer][one - 1][two + 1] * one / n;
    if (two > 0) d += dp[zer][one][two - 1] * two / n;
    d /= 1.0 - 1.0 * comp / n;

    dp[zer][one][two] = d;

    dump(zer << " " << one << " " << two << " => " << d)
  }

  int zer = 0, one = 0, two = 0;
  for (int i : range(n)) {
    if (vs[i] == 0) zer++;
    if (vs[i] == 1) one++;
    if (vs[i] == 2) two++;
  }

  return dp[zer][one][two];
}

}

int main(int argc, char** argv) {
  cout << fixed << setprecision(12);
  int testcase = 1;
  if (argc > 1) testcase = atoi(argv[1]);
  while (testcase--) {
    solver::read();
  }
  cout << solver::run() << endl;
}