#include <cstdio>
#include <algorithm>
using namespace std;

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;}};

double E[111][111][111];

int main(void) {
  int n; scanf("%d", &n);
  for(int k0 : range(n+1)) {
    for(int k1 : range(n+1)) {
      for(int k2 : range(n+1)) {
        int k = k0 + k1 + k2;
        if(k == 0) { continue; }
        E[k0][k1][k2] = double(n) / k;
        if(k0 > 0) { E[k0][k1][k2] += E[k0-1][k1+1][k2]   * k0 / k; }
        if(k1 > 0) { E[k0][k1][k2] += E[k0]  [k1-1][k2+1] * k1 / k; }
        if(k2 > 0) { E[k0][k1][k2] += E[k0]  [k1]  [k2-1] * k2 / k; }
      }
    }
  }
  vector<int> x(3);
  for(int i : range(n)) {
    int a; scanf("%d", &a);
    if(a < 3) { x[a]++; }
  }
  printf("%f\n", E[x[0]][x[1]][x[2]]);
  return 0;
}