#include<stdio.h>
#include<math.h>
int n,j,k,sol;
double stot,rasp,s[30];
void back(int l,int pi,double qq)
{
	if(l==k)
	{
		stot+=qq;
		sol++;
		return;
	}
	for(int i=pi+1;i<=n+l-k+1;i++)
        back(l+1,i,qq*s[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("%lf",&s[i]);
    back(0,0,1.0);
    rasp=(double)stot/(double)sol;
    printf("%.6lf",rasp*2);
	return 0;
}