//#include <iostream>
#include <queue>
#include <stack>
#include <map>
#include <cstdio>
#include <cmath>
#include <cstring>
#include <set>
#include <algorithm>
#include <bitset>
#include <time.h>
#include <tuple>
#include <fstream>
#include <iomanip>
#include <utility>

#pragma warning "da 100% din tine. :)"
#define nl '\n'
#define cnl cout << '\n';
#define sp ' '
#define pb(x) push_back(x)
#define all(v) v.begin(),v.end()
#define rall(v) v.rbegin(),v.rend()
#define SIZE 3000
#define NR 105
#define N 1300000
#define INF 10000
#define ll long long
#define ull unsigned ll
#define F "dezastru"

using namespace std;
ifstream cin(F".in");
ofstream cout(F".out");

//set, multiset, vector, unordered_set, list
//apelare print(nume_Variabila, (tip de date)1);
//ex: am un vector V de double
//print(V, (double) 1);
//ex: am un set de int-uri
//print(set, (int) 1);
template<class v, class type>
void print(v Vector, type nr) {
    for_each(all(Vector), [](type x) {
        cout << x << ' ';
    });
}

void nimic() {
    return;
}

vector<double> com;
vector<double> final;
double vals[26];
int n, k;

int ver[26];

ll f[26];

ll fact(int ord) {
    if (f[ord])
        return f[ord];
    if (ord == 1)
        return 1;
    return ord * fact(ord - 1);
}


void comb(int pas) {
    if (com.size() == k) {
        double produs = 1;
        for_each(all(com), [&produs](double k) {
            produs *= k;
        });
        final.emplace_back(produs);
    } else
        for (int i = 1; i <= n; ++i) {
            if (!ver[i]) {
                ver[i] = 1;
                com.emplace_back(vals[i]);
                comb(i);
                com.pop_back();
                ver[i] = 0;
            }
        }
}

int main() {
    ios_base::sync_with_stdio(false);
//    freopen("caps.in", "r", stdin);
//    freopen("caps.out", "w", stdout);
    clock_t tStart = clock();
    cin >> n >> k;
    for (int i = 1; i <= n; ++i)
        cin >> vals[i];
    comb(0);
    double rez = 0;
    for_each(all(final), [&rez](double z) {
        rez += z/final.size();
    });
    cout << setprecision(6) << fixed << rez;
    printf("\nTime taken: %.2fs\n", (double) (clock() - tStart) / CLOCKS_PER_SEC);
}