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#include<bits/stdc++.h>

using namespace std;

typedef double in;



void solve()
{
	int a,b;
	cin>>a>>b;
	vector<in>v(a);
	for(int z=0;z<a;z++)cin>>v[z];
	sort(v.begin(),v.end());
	
	vector<in> m(a*2+3,0);
	
	in wyn=0;
	m[1+a+1]=v.back();
	if(b==1)wyn=v.back();
	m[-1+a+1]=1-v.back();
	int i=1;
	for(int z=(int)v.size()-2;z>=0;z--)
	{
		i++;
		
		in po=v[z];
		in dop=1-po;
		vector<pair<int,in>>temp;
		in tem=0;
		for(int w=-i;w<=i;w++)
		{
			in pom=m[w+1+a+1]*dop+m[w-1+a+1]*po;
			if(w>=b)tem+=pom;
			temp.push_back({w+a+1,pom});
		}
		for(auto w:temp)m[w.first]=w.second;
		wyn=max(wyn,tem);
	}
	cout<<fixed<<setprecision(10)<<wyn;
}


int main()
{
	ios_base::sync_with_stdio(false);
	cin.tie(0);
	
	int h=1;
	while(h--)solve();
	
	return 0;
}