//#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;
}

double cumsaitidaiseamadeasaceva[30][30];

double vals[26];
int n, k;

double fact(int ord) {
    if (ord == 0)
        return 1;
    return ord * fact(ord - 1);
}

double rez;

int main() {
    ios_base::sync_with_stdio(false);
    clock_t tStart = clock();
    cin >> n >> k;
    for (int i = 1; i <= n; ++i)
        cin >> vals[i];
    for (int i = 0; i <= n; ++i)
        cumsaitidaiseamadeasaceva[i][0] = 1;
    for (int i = 1; i <= n; ++i)
        for (int j = 1; j <= i; ++j)
            cumsaitidaiseamadeasaceva[i][j] =
                    cumsaitidaiseamadeasaceva[i - 1][j] + cumsaitidaiseamadeasaceva[i - 1][j - 1] * vals[i];
    rez = cumsaitidaiseamadeasaceva[n][k] * fact(k) * fact(n - k) / fact(n);
    cout << setprecision(6) << fixed << rez;
    printf("\nTime taken: %.2fs\n", (double) (clock() - tStart) / CLOCKS_PER_SEC);
}