#include<cstdio>
#include<fstream>
#include<iostream>

using namespace std;

#define nn 30

int v[nn],x[nn];
float val[nn],s,pp;
int n,k;

void back (int p)
{

int i;
if(p>k){
pp=1;
for(int j=1;j<=k;++j)
pp*=val[v[j]];
s+=pp;
}
else
for(i=1;i<=n;++i)
if(x[i]==0){
v[p]=i;
x[i]=1;
back(p+1);
x[i]=0;
}

}

int main ()
{

ifstream fin ("dezastru.in");
freopen("dezastru.out","w",stdout);
fin>>n>>k;
for(int i=1;i<=n;++i)
fin>>val[i];
fin.close();

back(1);

for(int i=1;i<=n;++i)
s/=i;

printf("%f",s);

return 0;}