#pragma GCC optimize(2,3,"Ofast","unroll-loops")
#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define db double
#define in inline
namespace fast_io
{
	char buf[1<<12],*p1=buf,*p2=buf,sr[1<<23],z[23],nc;int C=-1,Z=0,Bi=0,ny;
	in char gc() {return p1==p2&&(p2=(p1=buf)+fread(buf,1,1<<12,stdin),p1==p2)?EOF:*p1++;}
	in ll read()
	{
		ll x=0;ny=1;while(nc=gc(),(nc<48||nc>57)&&nc!=EOF)if(nc==45)ny=-1;Bi=1;if(nc<0)return nc;
		x=nc-48;while(nc=gc(),47<nc&&nc<58&&nc!=EOF)x=(x<<3)+(x<<1)+(nc^48),Bi++;return x*ny;
	}
	in db gf() {int a=read(),y=ny,b=(nc!='.')?0:read();return (b?a+(db)b/pow(10,Bi)*y:a);}
	in int gs(char *s) {char c,*t=s;while(c=gc(),c<=32);*s++=c;while(c=gc(),c>32)*s++=c;return s-t;}
	in void ot() {fwrite(sr,1,C+1,stdout);C=-1;}
	in void flush() {if(C>1<<22) ot();}
	template <typename T>
	in void write(T x,char t)
	{
		int y=0;if(x<0)y=1,x=-x;while(z[++Z]=x%10+48,x/=10);
		if(y)z[++Z]='-';while(sr[++C]=z[Z],--Z);sr[++C]=t;flush();
	}
	in void write(char *s) {int l=strlen(s);for(int i=0;i<l;i++)sr[++C]=*s++;sr[++C]='\n';flush();}
};
using namespace fast_io;
#define pb push_back
const int N=55;
int n,c,l[N],p[N];ll f[N*6][N];
ll solve()
{
	for(int i=0;i<=n*c;i++) for(int s=0;s<(1<<(n-1));s++) f[i][s]=0;
	f[n*l[n]][0]=1;
	for(int i=0;i<n*c;i++) if((i%n))
	{
		int x=p[i%n];
		for(int s=0;s<(1<<(n-1));s++) if(!(s&(1<<(x-1))))
		for(int j=i;j<=n*c;j++) f[min(n*c,max(j,i+n*l[x]))][s|(1<<(x-1))]+=f[j][s];
	}
	return f[n*c][(1<<(n-1))-1];
}
int main()
{
	n=read(),c=read();for(int i=1;i<=n;i++) l[i]=read();
	sort(l+1,l+n+1);for(int i=1;i<n;i++) p[i]=i;
	db ans=0;
	do{ans+=solve();}while(next_permutation(p+1,p+n));
	for(int i=1;i<n;i++) ans/=i*c;printf("%.12lf",ans);
	return ot(),0;
}
//Author: disangan233