#include <cstdio>

using namespace std;

int x[26],viz[26],n,k,nr;
float p[26],s;

void citire ()  {     
	int i;     
	freopen("dezastru.in","r",stdin);     
	scanf("%d%d",&n,&k);     
	for (i=1;i<=n;i++)         
		scanf("%f",&p[i]);
	}   
void actualizare (float pr) {     
	s+=2*pr*p[x[k]]; }   

inline void back (int vf, float pr) {     
	int i;     
	for (i=x[vf-1]+1;i<=n-k+vf;i++) 
	{         
		x[vf]=i;         
		if (vf==k) {             
			nr+=2;             
			actualizare(pr);             
		}         
		else back(vf+1, pr*p[x[vf]]);        
	} 
}   
int main () {    
	citire();     
	back(1,1);     
	freopen ("dezastru.out","w",stdout);     
	printf("%.6f",s/nr); 
	return 0; 
}