#include <stdio.h>
#define FOR(s,e,i) for(i=(s);i<(e);++i)
double rez,a[26];
int nr,n,k1;
void back(int ant, int k, double s)
{
     if (k==k1+1)
	{ rez+=s; ++nr; }
     else 
	FOR (ant+1;n-(k1-k);int i) 
		back(i,k+1,s*a[i]);
}
int main()
{
	freopen ("dezastru.in","r",stdin);
 	freopen ("dezastru.out", "w",stdout);
 	scanf("%d%d", &n, &k1);
 	FOR(1;n;int i) 
		scanf("%lf", a+i);
	back(0,1,1);
 	rez/=nr;
 	printf("%lf", rez);
	return 0;
}