#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
using namespace std;
void openIOFiles()
{
 freopen("dezastru.in","r",stdin);
 freopen("dezastru.out","w",stdout);
}
vector<float>V;
float produs(1),sol(0),fact(1),aux;
int i,j,n,St[30],p;
int main()
{
 openIOFiles();
 scanf("%d%d",&n,&p);
 for (i=1;i<=n;++i) {
                     scanf("%f",&aux);
                     V.push_back(aux);
                     }
 do{
    produs=1;
    for (i=0;i<p;++i) produs*=V[i]; 
    sol+=produs;
    }
 while(next_permutation(V.begin(),V.end()));
 for (i=1;i<=n;++i) fact*=i;
 printf("%f",sol/fact);
 return 0;
}