#include<cstdio>
using namespace std;
int d1(int a)
{
	if(!a)
		return 0;
	return 1+d1(a&(a-1));
}
int main()
{
	freopen("dezastru.in","r",stdin);freopen("dezastru.out","w",stdout);
	float v[26],v1[200000],s=0;int i,j,n,k,bit,index=0;
	scanf("%d%d",&n,&k);
	bit=(1<<n)-1;
	for(i=1;i<=n;i++)
		scanf("%f",&v[i]);
	for(i=1;i<=bit;i++)
	{
		if(d1(i)!=k)
			continue;
		v1[++index]=1;
		for(j=0;j<n;j++)
			if((i>>j)&1)
				v1[index]*=v[j+1];
	}
	for(i=1;i<=index;i++)
		s+=v1[i]/index;
	printf("%f",s);
}