#include<iostream>
#include<cstdio>
#define N 4000
using namespace std;
int read()
{
    char c=0;
    int sum=0;
    while (c<'0'||c>'9') c=getchar();
    while ('0'<=c&&c<='9') sum=sum*10+c-'0',c=getchar();
    return sum;
}
struct reads
{
    double d[N+1];
};
reads nw,e,res,c;
int n,L,a[N+1];
double p[N+1],s[N+1],dp[N+1],DP[N+1],delta[N+1],ans[N+1],tans;
reads operator * (reads a,reads b)
{
    for (int i=0;i<=3998;++i) c.d[i]=0;
    for (int i=0;i<=1999;++i)
	for (int j=0;j<=1999;++j)
	    c.d[i+j]+=a.d[i]*b.d[j];
    for (int i=3998;i>=2000;--i)
    {
	for (int k=1;k<=2000;++k) c.d[i-k]+=c.d[i]*p[k];
	c.d[i]=0;
    }
    return c;
}
reads F(reads x)
{
    c.d[0]=0;
    for (int i=1;i<=2000;++i) c.d[i]=x.d[i-1];
    for (int i=1;i<=2000;++i) c.d[2000-i]+=c.d[2000]*p[i];
    c.d[2000]=0;
    return c;
}
reads fast_pow(reads a,int b)
{
    reads res=e,mul=a;
    while (b)
    {
	if (b&1) res=res*mul;
	mul=mul*mul,b>>=1;
    }
    return res;
}
int main()
{
    double rst=0;
    n=read(),L=read(),dp[0]=nw.d[1]=e.d[0]=1;
    for (int i=1;i<=n;++i) a[i]=read(),p[a[i]]+=1.0/n;
    for (int i=1;i<=1999;++i)
	for (int j=1;j<=i;++j)
	    dp[i]+=dp[i-j]*p[j];
    if (L<=1999) DP[L]=1;
    else
    {
	res=fast_pow(nw,max(L-3999,0));
	for (int i=0;i<=1999;++i) DP[min(L,3999)]+=res.d[i]*dp[i];
	for (int i=max(L-3999,0)+1;i<=L-2000;++i)
	{
	    res=F(res);
	    for (int j=0;j<=1999;++j) DP[L-i]+=res.d[j]*dp[j];
	}
        for (int i=0;i<=1999;++i)
	    for (int j=2000;j<=min(i+2000,min(L,3999));++j)
		DP[i]+=DP[j]*p[j-i];
    }
    for (int i=0;i<=min(L,1999);++i) s[i]=DP[i];
    for (int i=min(L,1999);i>=0;--i)
    {
	delta[i]=1;
	for (int j=1;j<=i;++j) s[i-j]+=s[i]*p[j];
	for (int j=0;j<=i-1;++j) delta[i]-=s[j];
    }
    for (int i=0;i<=min(L,1999);++i)
    {
	ans[i]=delta[i],rst=0;
	for (int j=1;j<=i;++j) rst+=ans[i-j]*p[j];
	ans[i]=max(ans[i],rst),tans+=ans[i]*DP[i];
    }
    printf("%0.12lf\n",tans);
    return 0;
}