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#include <cstdio>
#include <utility>
#include <cmath>
#include <algorithm>
#include <functional>

using ll = long long;

using namespace std;

const int N = 50005;

double dp[N][2];

double p[N];

int main() { 
    int n, t;
    scanf("%d%d", &n, &t);
    for (int i = 1; i <= n; i++) {
        scanf("%lf", &p[i]);
    }
    sort(p + 1, p + n + 1, greater<double>());

    dp[0][0] = 1;
    int must_have_n = t + (n - t + 1) / 2;

    for (int i = 1; i < t; i++) {
        int least_calc = max(1, must_have_n - n + i);

        int i_mod = i % 2;
        int i_dec_mod = 1 - i_mod;
        
        dp[0][i_mod] = dp[0][i_dec_mod] * (1 - p[i]);
        for (int j = least_calc; j <= i; j++) {
            dp[j][i_mod] = p[i] * dp[j - 1][i_dec_mod] + (1 - p[i]) * dp[j][i_dec_mod];
        }
    }

    double best_res = 0;
    double cnd_res = 0;

    for (int i = t; i <= n; i++) {
        int must_have = t + (i - t + 1) / 2;
        int least_calc = max(1, must_have_n - n + i);

        int i_mod = i % 2;
        int i_dec_mod = 1 - i_mod;


        dp[0][i_mod] = dp[0][i_dec_mod] * (1 - p[i]);
        for (int j = least_calc; j < must_have_n; j++) {
            dp[j][i_mod] = p[i] * dp[j - 1][i_dec_mod] + (1 - p[i]) * dp[j][i_dec_mod];
        }
        
        if ((i - t) % 2 == 0) {
            cnd_res += p[i] * dp[must_have - 1][i_dec_mod];
        } else {
            cnd_res -= (1 - p[i]) * dp[must_have - 1][i_dec_mod];
        }
        
        best_res = max(best_res, cnd_res);
    }

    printf("%.7lf\n", best_res);
    return 0;
}