// while (clock()<=69*CLOCKS_PER_SEC)
// #pragma comment(linker, "/stack:200000000")
// #pragma GCC optimize("O3")
// #pragma GCC target ("avx2")
// #pragma GCC optimize("Ofast")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
// #pragma GCC optimize("unroll-loops")
// #include <bits/stdc++.h>
// #include <ext/pb_ds/assoc_container.hpp>
// #include <ext/pb_ds/tree_policy.hpp>
// #include <cmath>
// #include <functional>
// #include <fstream>
// #include <algorithm>
// #include <string>
// #include <set>
// #include <map>
// #include <list>
// #include <time.h>
// #include <math.h>
// #include <random>
// #include <deque>
// #include <queue>
// #include <cassert>
// #include <unordered_map>
// #include <unordered_set>
// #include <iomanip>
// #include <bitset>
// #include <sstream>
// #include <chrono>
// #include <cstring>
///#include <vector>
#include <cstdio>
#include <iostream>
#include <iomanip>
#include <algorithm>

using namespace std;

typedef long long ll;
typedef long double ld;
///mt19937 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
bool HOME = 1;
const string TASK_NAME = "dezastru";
#define files(TASK_NAME) read(TASK_NAME); write(TASK_NAME)
#define read(TASK_NAME) freopen((TASK_NAME + ".in").c_str(), "r", stdin)
#define write(TASK_NAME) freopen((TASK_NAME + ".out").c_str(), "w", stdout)
#define all(c) (c).begin(), (c).end()
#define sz(c) (int) (c).size()
#define left left228
#define right right228
#define y1 y1228
#define mp make_pair
#define pb push_back
#define y2 y2228
#define rank rank228

const int N = 30;
int n, k;
ld a[N], mlt, sol;

void dfs(int pos, int cur, ld prod) {
  if (cur > k) {
    return;
  }
  if (pos == n + 1) {
    if (cur == k) {
      sol += prod;
    }
    return;
  }
  dfs(pos + 1, cur, prod);
  dfs(pos + 1, cur + 1, prod * a[pos]);
}

int main() {
#ifdef INFOARENA
    HOME = 0;
#endif
  if (HOME) {
    freopen ("input", "r", stdin);
  } else {
    files(TASK_NAME);
  }
  cin >> n >> k;
  for (int i = 1; i <= n; i++) {
    cin >> a[i];
  }
  mlt = 1.0;
  for (int i = 1; i <= k; i++) {
    mlt *= i;
  }
  for (int i = 1; i <= n; i++) {
    mlt /= i;
  }
  for (int i = 1; i <= n - k; i++) {
    mlt *= i;
  }
  dfs(1, 0, 1);
  cout << fixed << setprecision(6) << mlt * sol << "\n";
  return 0;
}