#include <iostream>
#include <vector>
#include <cmath>
#include <cstring>
#include <algorithm>
#include <iomanip>
using namespace std;
#define FastIO cin.tie(nullptr) -> sync_with_stdio(false);
#define Cases int __; cin >> __; for (int _ = 1; _ <= __; _ ++)
#define endl '\n'

typedef long long ll;
typedef pair<int, int> pii;
const int mod = 1e9 + 7;
// const int mod = 998244353;
const int N = 5e3 + 2;
const int inf = 0x3f3f3f3f;
int n, m;
int k;
double p[N];
double a[N][N << 1];

void solve() {
    cin >> n >> k;
    for (int i = 1; i <= n; i ++) cin >> p[i];
    sort(p + 1, p + n + 1);
    reverse(p + 1, p + n + 1);
    double res = 0;
    a[0][N] = 1;
    for (int i = 1; i <= n; i ++) {
        for (int j = -i; j <= i; j ++)
            a[i][N + j] = a[i - 1][N + j - 1] * p[i] + a[i - 1][N + j + 1] * (1 - p[i]);
        double t = 0;
        for (int j = k; j <= n; j ++) t += a[i][N + j];
        res = max(res, t);
    }
    printf("%.8lf\n", res);
}

int main() {
    solve();
    return 0;
}