#include <fstream>
using namespace std;

ifstream cin("dezastru.in");
ofstream cout ("dezastru.out");

int x[26];
float a[26];
int n,k;
float sum,cnt = 0;
bool pus[25];

void afis(){
    float prod= 1;
    for (int i = 1; i <=k;++i)
    prod*= a[x[i]];
    sum += prod;
    ++cnt;
}

void back(int pas){
    for (int i = 1; i <= n;++i)
    {
        if (!pus[i])
            {
            x[pas] = i;
            pus[i] =1;
            
        if (pas < k)
        back(pas+1);
        else
        afis();
        pus[i] = 0;
        }
    }
    
}

int main(){
    cin >> n >> k;
    for (int i = 1; i<=n;++i)
    cin >> a[i];
    back(1);
    cout << sum / cnt;
}