// #pragma GCC target("avx")  // CPU 処理並列化
// #pragma GCC optimize("O3")  // CPU 処理並列化
// #pragma GCC optimize("unroll-loops")  // 条件処理の呼び出しを減らす
// #define BEGIN_STACK_EXTEND(size) void * stack_extend_memory_ = malloc(size);void * stack_extend_origin_memory_;char * stack_extend_dummy_memory_ = (char*)alloca((1+(int)(((long long)stack_extend_memory_)&127))*16);*stack_extend_dummy_memory_ = 0;asm volatile("mov %%rsp, %%rbx\nmov %%rax, %%rsp":"=b"(stack_extend_origin_memory_):"a"((char*)stack_extend_memory_+(size)-1024));
// #define END_STACK_EXTEND asm volatile("mov %%rax, %%rsp"::"a"(stack_extend_origin_memory_));free(stack_extend_memory_);

#include<stdio.h>
#include<math.h>
#include<algorithm>
#include<queue>
#include<deque>
#include<stack>
#include<string>
#include<string.h>
#include<vector>
#include<set>
#include<map>
#include<bitset>
#include<stdlib.h>
#include<cassert>
#include<time.h>
#include<bitset>
#include<numeric>
#include<unordered_set>
#include<unordered_map>
#include<complex>
using namespace std;
const long long mod=1000000007;
const long long inf=mod*mod;
const long long d2=(mod+1)/2;
const double EPS=1e-11;
const double INF=1e+10;
const double PI=acos(-1.0);
const int C_SIZE = 3100000;
const int UF_SIZE = 3100000;
namespace{
	long long fact[C_SIZE];
	long long finv[C_SIZE];
	long long inv[C_SIZE];
	inline long long Comb(int a,int b){
	 	if(a<b||b<0)return 0;
	 	return fact[a]*finv[b]%mod*finv[a-b]%mod;
	}
	void init_C(int n){
		fact[0]=finv[0]=inv[1]=1;
		for(int i=2;i<n;i++){
			inv[i]=(mod-(mod/i)*inv[mod%i]%mod)%mod;
		}
		for(int i=1;i<n;i++){
			fact[i]=fact[i-1]*i%mod;
			finv[i]=finv[i-1]*inv[i]%mod;
		}
	}
	long long pw(long long a,long long b){
		if(a<0LL)return 0;
		if(b<0LL)return 0;
		long long ret=1;
		while(b){
			if(b%2)ret=ret*a%mod;
			a=a*a%mod;
			b/=2;
		}
		return ret;
	}
	long long pw_mod(long long a,long long b,long long M){
		if(a<0LL)return 0;
		if(b<0LL)return 0;
		long long ret=1;
		while(b){
			if(b%2)ret=ret*a%M;
			a=a*a%M;
			b/=2;
		}
		return ret;
	}
	int pw_mod_int(int a,int b,int M){
		if(a<0)return 0;
		if(b<0)return 0;
		int ret=1;
		while(b){
			if(b%2)ret=(long long)ret*a%M;
			a=(long long)a*a%M;
			b/=2;
		}
		return ret;
	}
	int ABS(int a){return max(a,-a);}
	long long ABS(long long a){return max(a,-a);}
	double ABS(double a){return max(a,-a);}
	int sig(double r) { return (r < -EPS) ? -1 : (r > +EPS) ? +1 : 0; }
	int UF[UF_SIZE];
	void init_UF(int n){
		for(int i=0;i<n;i++)UF[i]=-1;
	}
	int FIND(int a){
		if(UF[a]<0)return a;
		return UF[a]=FIND(UF[a]);
	}
	void UNION(int a,int b){
		a=FIND(a);b=FIND(b);if(a==b)return;
		if(UF[a]>UF[b])swap(a,b);
		UF[a]+=UF[b];UF[b]=a;
	}
}
// ここから編集しろ
int p[10];
int perm[6];
double dp[60][1<<5][110][10];
int main(){
	int a,b;scanf("%d%d",&a,&b);
	for(int i=0;i<a;i++)scanf("%d",p+i);
	std::sort(p,p+a);
	int n=a-1;
	for(int i=0;i<n;i++){
		perm[i]=i+1;
	}
	double ret=0;
	do{
		for(int i=0;i<=b;i++)for(int j=0;j<(1<<n);j++)for(int k=0;k<110;k++)for(int l=0;l<10;l++){
			dp[i][j][k][l]=0;
		}
		dp[0][0][p[n]][0]=1;
		for(int i=0;i<b;i++){
			for(int j=0;j<(1<<n);j++){
				for(int k=i;k<110;k++){
					for(int l=0;l<a;l++){
						if(dp[i][j][k][l]<EPS)continue;
						for(int m=0;m<(1<<n);m++){
							if(j&m)continue;
							double ks=1;
							int fr=10;
							int tk=k;
							int tl=l;
							for(int x=0;x<n;x++){
								if(j&(1<<x))continue;
								if(m&(1<<x)){
									ks*=1.0/(b-i);
									fr=min(fr,perm[x]);
									if(tk<i+p[x]||(tk==i+p[x]&&tl<perm[x])){
										tk=i+p[x];
										tl=perm[x];
									}
								}else{
									ks*=1.0-(1.0)/(b-i);
								}
							}
							int tj=j+m;
							if(fr>l&&k==i)continue;
							
							dp[i+1][tj][tk][tl]+=ks*dp[i][j][k][l];
						}
					}
				}
			}
		}
		for(int i=b;i<110;i++){
			for(int j=0;j<10;j++){
				ret+=dp[b][(1<<n)-1][i][j];
			}
		}
	}while(next_permutation(perm,perm+n));
	for(int i=1;i<=n;i++)ret/=i;
	printf("%.18f\n",ret);
}


Measured memory_bytes: 14464