#include <cstdio>
#include <cstdlib>
int n,k;
float v[26];
int st[30],is[30];
int found_sol;

void  back (int p)
{
	int i,c;
	float s;
	if(p==k+1)
	{
		s = 0;
	for (i=1;i<=n;i++)
		s=s+v[i];
	printf("%f\n",s /(float) n);
	exit(0);
	}
	else 
	{
		for (c=1;c<=n;c++)
			if (!is[c])
			{
				st[p]=c;
				is[c]=1;
				back(p+1);
				is[c]=0;
			}
	}
}
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",&v[i]);
	back (1);
	return 0;
}