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

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

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 *)malloc((n+1)*(n+1)*(n+1)*sizeof(double));
  e[POS(0,0,0)]=0.0;

  double p=(double)1/n;
  int j,k;
  for(i=0;i<=cnt[0];i++){
    for(j=0;j<=cnt[1]+cnt[0]-i;j++){
      for(k=(i==0&&j==0);k<=cnt[2]+cnt[1]+cnt[0]-i-j;k++){
	e[POS(i,j,k)]=(1.0+i*p*(i>0?e[POS(i-1,j+1,k)]:0)+j*p*(j>0?e[POS(i,j-1,k+1)]:0)+k*p*(k>0?e[POS(i,j,k-1)]:0))*n/(i+j+k);
      }
    }
  }

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

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