#include<bits/stdc++.h>
using namespace std;
#define ll long long
#define llu unsigned long long
#define ld double
#define llu unsigned long long
#define rep(i,x,y)  for(ll i=int(x);i<int(y);++i)
#define For(i,x,y)  for(ll i=int(x);i<=int(y);++i)
#define FOr(i,x,y)  for(ll i=int(x);i>=int(y);--i)
#define pi acos(-1) 
#define mk make_pair
#define pa pair<ll,ll>
#define lf else if
#define IL inline
#define sqr(x)      ((x)*(x))
#define Mul(x,y)    ((x)=1LL*(x)*(y)%mod)
#define Add(x,y)    ((x)=((x)+(y))%mod)
#define E(x)        return writeln(x),0
#define LL          (long long)
#define p(x)        printf("~%lld~\n",LL(x))
#define pp(x,y)     printf("~~%lld %lld~~\n",LL(x),LL(y))
#define ppp(x,y,z)  printf("~~~%lld %lld %lld~~~\n",LL(x),LL(y),LL(z))
#define pppp(a,b,c,d)   printf("~~~%lld %lld %lld %lld\n",LL(a),LL(b),LL(c),LL(d))
#define f_in(x)     freopen(x".in","r",stdin)
#define f_out(x)    freopen(x".out","w",stdout)
#define open(x)     f_in(x),f_out(x)
#define fi first
#define se second
#define y1          fafa____
typedef complex<double> E;
namespace SHENZHEBEI{
#ifdef LOCAL
    struct _{_(){freopen("cf.in","r",stdin);}}_; 
#endif 
#define NEG 1
    static const int GYN=2333333;
    char SZB[GYN],*SSS=SZB,*TTT=SZB;
    inline char gc(){   if (SSS==TTT){  TTT=(SSS=SZB)+fread(SZB,1,GYN,stdin); if (SSS==TTT) 

return '\n';  }   return *SSS++;    }
#define gc getchar
#if NEG
    inline ll read(){    ll x=0,g=1; char ch=gc();   for (;!isdigit(ch);ch=gc()) if 

(ch=='-') g=-1;  for (;isdigit(ch);ch=gc())  x=x*10-48+ch;   return x*g; }
    inline void write(ll x){    if (x<0) putchar('-'),x=-x;  if (x>=10)   write(x/10);    

putchar(x%10+'0');  }
#else
    inline ll read(){    ll x=0; char ch=gc();   for (;!isdigit(ch);ch=gc());    for 

(;isdigit(ch);ch=gc())  x=x*10-48+ch;   return x;   }
    inline void write(ll x){    if (x>=10)   write(x/10);    putchar(x%10+'0');  }
#endif
    inline char readchar(){ char ch=gc();   for(;isspace(ch);ch=gc());  return ch;  }
    inline ll readstr(char *s){ char ch=gc();   int cur=0;  for(;isspace(ch);ch=gc());      

for(;!isspace(ch);ch=gc())  s[cur++]=ch;    s[cur]='\0';    return cur; }
    void Print(long long *a,int s,int t){For(i,s,t)printf("%lld ",a[i]);puts("");}
    void Print(int *a,int s,int t){For(i,s,t)printf("%d ",a[i]);puts("");}
    void Print(char *a,int s,int t){For(i,s,t)putchar(a[i]);puts("");}
    void writeln(ll x){write(x),puts("");}
    void Max(ll &x,ll y){x=x<y?y:x;}
    ll max(ll x,ll y){return x<y?y:x;}
}using namespace SHENZHEBEI;
const ll N=10;
ll ans,gg,all,n,C,f[400][40],a[40],b[40];
void work(){
	memset(f,0,sizeof f);
	f[a[n]*n][0]=1;
	rep(i,0,n*C)For(j,i,n*C)
	For(S,0,all){
		if (!(i%n))continue;
		if (S>>b[i%n-1]&1)continue;
		f[min(n*C,max(j,i+n*a[b[i%n-1]+1]))][S|(1<<b[i%n-1])]+=f[j][S];
	}ans+=f[n*C][all];++gg;
//	writeln(f[n*C][all]);
}
void dfs(ll x){
	if (x==n-1)return work(),void(0);
	For(i,0,n-2)if (b[i]==-1){
		b[i]=x;
		dfs(x+1);
		b[i]=-1;
	}
}
int main(){
	n=read();C=read();all=(1<<n-1)-1;
	For(i,1,n)a[i]=read();
	sort(a+1,a+n+1);
	memset(b,-1,sizeof b);
	dfs(0);
	For(i,1,n-1)gg*=C;
	printf("%.12lf\n",1.0*ans/gg);
}
/*
[						]
	[						]
		[		]
			[		]	
*/

Measured memory_bytes: 384