#include<fstream>
#include<algorithm>
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int n,i,k,nr,x[30];
float s,p,a[26];
int main()
{f>>n>>k;
for(i=1;i<=n;++i)
	f>>a[i];
for(i=1;i<=n;++i)
	x[i]=i;
p=1;
for(i=1;i<=k;++i)
	p=p*a[i];
s=p;
nr=1;

while(next_permutation(x+1,x+n+1))
{p=1;
++nr;
for(i=1;i<=k;++i)
	p=p*a[x[i]];
s+=p;
}
g.precision(19);
g<<s/nr<<'\n';
return 0;
}