#include<cstdio>
using namespace std;
int d1(int a)
{
	if(!a)
		return 0;
	return 1+d1(a&(a-1));
}
float v[500],v1[90000000],s=0;
int main()
{
	freopen("dezastru.in","r",stdin);freopen("dezastru.out","w",stdout);
	int i,j,n,k,bit,index=0;
	scanf("%d%d",&n,&k);
	bit=(1<<n)-1;
	for(i=0;i<n;i++)
		scanf("%f",&v[1<<i]);
	for(i=1;i<=bit;i++)
	{
		if(d1(i)!=k)
			continue;
		v1[++index]=1;
		for(j=i;j;j&=(j-1))
			v1[index]*=v[j^(j&(j-1))];
	}
	for(i=1;i<=index;i++)
		s+=v1[i]/index;
	printf("%f",s);
}