#pragma GCC optimize("Ofast")
#pragma GCC target("avx2")

#include<unistd.h>
#include<sys/mman.h>

#define rd() ({int _v=0,_c;while(_c=*rp++-48,_c>=0)_v=_v*10+_c;_v;})
#define wt(v) ({unsigned _z=v;do*--wp=_z%10+48;while(_z/=10);})
#define rep(v,e) for(int v=0;v<e;++v)
#define rep3(v,s,e) for(int v=s;v<e;++v)
#define e(a,b,c) _e[a+1][b+1][c+1]

constexpr struct Hoge {
	double _e[102][102][102];
	constexpr Hoge():_e(){
		rep(i,101){
			rep3(j,i,101){
				rep3(k,j?:1,101){
					e(i,j,k)=(i*e(i-1,j,k)+(j-i)*e(i,j-1,k)+(k-j)*e(i,j,k-1)+1)/k;
				}
			}
		}
	}
} h;

static double f1(){
	char*rp=static_cast<char*>(mmap(0,1<<25,1,2,0,0));
	int n=rd();
	int c0=0,c1=0,c2=0;
	while(*rp){
		int a=rd();
		c0+=a<=0;
		c1+=a<=1;
		c2+=a<=2;
	}
	return n*h.e(c0,c1,c2);
}

static void f2(double z){
	char wbuf[64],*wp=wbuf+sizeof wbuf;
	*--wp='6';
	*--wp='-';
	*--wp='e';
	wt(z*1e6);
	write(1,wp,wbuf+sizeof wbuf-wp);
}

int main(){
	f2(f1());
	_exit(0);
}