#include <stdio.h>
double v[26],rez;
int nr,n,kk;
void back(int st,int k,double s)
{
     if (k==kk+1)
		{rez+=s;++nr;}
     else for (int i=st+1;i<=n-(kk-k);i++) 
	bk(i,k+1,s*v[i]);
}
int main()
{
 freopen ("dezastru.in","r",stdin);
 freopen ("dezastru.out", "w",stdout);
 scanf("%d%d",&n,&kk);
 for (int i=1;i<=n;++i) 
	scanf("%lf", v+i);
 back(0,1,1);
 rez/=nr;
 printf("%lf", rez);
 return 0;
}