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#include <bits/stdc++.h>
using namespace std;
#define rep(i,a,n) for (int i=(a);i<(n);i++)
#define per(i,a,n) for (int i=(n)-1;i>=(a);i--)
#define pb push_back
#define eb emplace_back
#define mp make_pair
#define all(x) (x).begin(),(x).end()
#define fi first
#define se second
#define SZ(x) ((int)(x).size())
typedef vector<int> VI;
typedef basic_string<int> BI;
typedef long long ll;
typedef pair<int,int> PII;
typedef long double db;
mt19937 mrand(1111); 
const ll mod=1000000007;
int rnd(int x) { return mrand() % x;}
ll powmod(ll a,ll b) {ll res=1;a%=mod; assert(b>=0); for(;b;b>>=1){if(b&1)res=res*a%mod;a=a*a%mod;}return res;}
ll gcd(ll a,ll b) { return b?gcd(b,a%b):a;}
// head

const int N=201000;
vector<db> mul(vector<db> a,vector<db> b) {
	vector<db> c(SZ(a)+SZ(b)-1,0);
	VI ib;
	rep(j,0,SZ(b)) if (fabs(b[j])>1e-18) ib.pb(j);
	rep(i,0,SZ(a)) if (fabs(a[i])>1e-18) {
		for (auto j:ib) c[i+j]+=a[i]*b[j];
	}
	return c;
}

db ret[N],p[N];
int n,k;
vector<db> solve(int l,int r,vector<db> prob,int vl) {
	int cl=0,cr=SZ(prob)-1;
	int len=r-l+1;
	while (cl<=cr&&2*(vl+cl+len)-r<k) cl++;
	db pwin=0;
	while (cl<=cr&&2*(vl+cr)-r>=k) pwin+=prob[cr],--cr;
	rep(i,l,r+1) ret[i]+=pwin;
	vl=vl+cl;
	//assert(cl<=cr);
	auto nprob=vector<db>(prob.begin()+cl,prob.begin()+cr+1);
	prob=nprob;
	if (l==r) {
		auto pr=vector<db>{1-p[l],p[l]};
		auto nprob=mul(prob,pr);
		//for (auto x:nprob) cout<<x<<" "; cout<<vl<<"!!!\n";
		rep(i,0,SZ(nprob)) if (2*(vl+i)-r>=k) ret[l]+=nprob[i];
		return pr;
	}
	int md=(l+r)>>1;
	auto pl=solve(l,md,prob,vl);
	prob=mul(prob,pl);
	auto pr=solve(md+1,r,prob,vl);
	return mul(pl,pr);
}

int main() {
	ios::sync_with_stdio(false);
	cin.tie(nullptr);
	cin>>n>>k;
	rep(i,1,n+1) cin>>p[i];
	sort(p+1,p+n+1); reverse(p+1,p+n+1);
	solve(1,n,vector<db>{1},0);
	db ans=*max_element(ret+1,ret+n+1);
	cout<<setiosflags(ios::fixed)<<setprecision(12)<<ans<<"\n";
}