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#include<bits/stdc++.h>
using namespace std;
#define ll long long
inline int read(){
   int s=0,w=1;
   char ch=getchar();
   while(ch<'0'||ch>'9'){if(ch=='-')w=-1;ch=getchar();}
   while(ch>='0'&&ch<='9') s=s*10+ch-'0',ch=getchar();
   return s*w;
}
#define ld long double
const ld pi=acos(-1);
struct cp
{
    ld r,i;
    cp operator+(const cp &x)const{return (cp){x.r+r,x.i+i};}
    cp operator-(const cp &x)const{return (cp){r-x.r,i-x.i};}
    cp operator*(const cp &x)const{return (cp){r*x.r-i*x.i,r*x.i+i*x.r};}
}A[1<<20],B[1<<20];
int rev[1<<20],N=1;
void init()
{
    for(int i=1,d=N>>1; i<=N; i<<=1,d>>=1)
        for(int j=0; j<i; ++j) rev[i+j]=rev[j]|d;
    return ;
}
void FFT(cp* F,int op)
{
    for(int i=0; i<N; ++i) if(rev[i]>i) swap(F[i],F[rev[i]]);
    for(int len=2,M=1; len<=N; len<<=1,M<<=1)
    {
        cp w=(cp){cos(pi/M*op),sin(pi/M*op)};
        //omega(m,1)
        for(int l=0,r=len-1; l<=N; l+=len,r+=len)
        {
            cp w0=(cp){1,0};
            for(int i=l; i<l+M; ++i)
            {
                cp x=F[i]+w0*F[i+M],y=F[i]-w0*F[i+M];
                F[i]=x,F[i+M]=y,w0=w0*w;
            }
        }
    }
}
ld a[50003],pre[50003];
vector<ld> conv(vector<ld> a,vector<ld> b)
{
	int sa=a.size(),sb=b.size(),sc=sa+sb-1;
	vector<ld> c(sc);
	if(sc<=64)
	{
		for(int i=0; i<sa; ++i)
			for(int j=0; j<sb; ++j)
				c[i+j]+=a[i]*b[j];
		return c;
	}
	for(N=1; N<sc; N<<=1);
	for(int i=0; i<N; ++i) A[i]={0,0};
	for(int i=0; i<N; ++i) B[i]={0,0};
	for(int i=0; i<sa; ++i) A[i].r=a[i];
	for(int i=0; i<sb; ++i) B[i].r=b[i];
	init(),FFT(A,1),FFT(B,1);
    for(int i=0; i<N; ++i) A[i]=A[i]*B[i];
    FFT(A,-1);
    ld inv_n=1.0L/N;
    for(int i=0; i<sc; ++i) c[i]=A[i].r*inv_n;
	return c;
}
vector<ld> solve(int l,int r,vector<ld> q)
{
	if(l==r)
	{
		int sz=q.size();
		for(int i=0; i<sz; ++i)
			if(i-1-sz/2>=0) pre[l]+=q[i];
			else if(i+1-sz/2>=0) pre[l]+=q[i]*a[l];
		return {1-a[l],0,a[l]};
	}
	int len=r-l+1;
	vector<ld> p(len*2+3);
	int sz=q.size();
	for(int i=0; i<sz; ++i)
		p[min(len*2+2,max(0,i-sz/2+len+1))]+=q[i];
	int mid=(l+r)>>1;
	auto pl=solve(l,mid,p);
	auto pr=solve(mid+1,r,conv(p,pl));
	return conv(pl,pr);
}
signed main()
{
	ios::sync_with_stdio(0),
    cin.tie(0),cout.tie(0);
	int n,m;
	cin>>n>>m;
	for(int i=1; i<=n; ++i) cin>>a[i];
	sort(a+1,a+n+1),reverse(a+1,a+n+1);
	vector<ld> sdt(m*2+1);
	sdt[0]=1,solve(1,n,sdt);
	ld ans=0;
	// for(int i=1; i<=n; ++i)
		// printf("%.10Lf\n",pre[i]);
	for(int i=1; i<=n; ++i) ans=max(ans,pre[i]);
	printf("%.10Lf\n",ans);
	return 0;
}