#include<stdio.h>

const int N_max=26;
int n,k,st[N_max],viz[N_max],nr=0;
float pb[N_max],sum=0;

void bkt(int p,int l,float prod)
{
	int i;
	if(p==k+1)
	{
		nr++;
		sum+=prod;
		return ;
	}
	for(i=l+1;i<=n-k+p;i++)
		bkt(p+1,i,prod*pb[i]);	

}
int main()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%d%d",&n,&k);
	for(int i=1;i<=n;++i)
		scanf("%f",&pb[i]);
	bkt(1,0,1);

	printf("%f\n",sum/nr);
	return 0;
}