#include<stdio.h>
#include<stdlib.h>

#define POS(i,j,k) (((i)*(n+1)+(j))*(n+1)+(k)) 

double calc(double *e,int n,int i,int j,int k){
  if(i==0 && j==0 && k==0) return 0.0;
  if(e[POS(i,j,k)]>=1.0) return e[POS(i,j,k)];

  double t=1.0;
  if(i>0) t+=(double)i/n*calc(e,n,i-1,j+1,k);

  if(j>0) t+=(double)j/n*calc(e,n,i,j-1,k+1);

  if(k>0) t+=(double)k/n*calc(e,n,i,j,k-1);

  t/=1-(double)(n-i-j-k)/n;

  e[POS(i,j,k)]=t;
  return t;
}

void run(void){
  int n;
  scanf("%d",&n);

  int cnt[4]={0};
  int i;
  for(i=0;i<n;i++){
    int a;
    scanf("%d",&a);
    cnt[a>=3?3:a]++;
  }
  double *e=(double *)calloc((n+1)*(n+1)*(n+1),sizeof(double));
  calc(e,n,cnt[0],cnt[1],cnt[2]);

  printf("%.6lf\n",e[POS(cnt[0],cnt[1],cnt[2])]);
  free(e);
  return;
}

int main(void){
  run();
  return 0;
}