#include<cstdio>
#define N 26
short int n,k,sol[N],inceput,t;
double v[N],g=1;
bool viz[N];
void citire()
{
	freopen("dezastru.in","r",stdin);
	freopen("dezastru.out","w",stdout);
	scanf("%hd%hd",&n,&k);
	for (short int i=1; i<=n; ++i)
		scanf("%lf",&v[i]);
}
void back(short int p,short int pas,double g)
{
	if (p==k+1)
	{
		v[0]+=g;
		++t;
		return;
	}
	for (int i=pas+1; i<=n-(k-p); ++i)
	{
		back(p+1,i,g*v[i]);
	}
}
int main()
{
	citire();
	back(1,0,1);
	v[0]/=t;
	printf("%.6lf",v[0]);
	return 0;
}