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#include <bits/stdc++.h>
using namespace std;
double dp[50123];
int main() {
	int n, t;
	scanf("%d%d", &n, &t);
	vector<double> ps(n);
	for (double& x : ps) {
		scanf("%lf", &x);
	}
	double answer = 0;
	sort(ps.rbegin(), ps.rend());
	dp[1] = 1;
	int low = 1, high = 1;
	for (int cnt = 1; cnt <= n; cnt++) {
		double p = ps[cnt-1];
		for (int i = high + 1; i >= low; i--) {
			dp[i] = dp[i] * (1 - p) + dp[i-1] * p;
		}
		high++;
		double maybe = 0;
		for (int i = low - 1; i <= high - 1; i++) {
			if (i - (cnt - i) >= t) {
				maybe += dp[i+1];
			}
		}
		answer = max(answer, maybe);
		if (high - low == 4000) {
			if (dp[low] < dp[high]) {
				low++;
			}
			else {
				high--;
			}
		}
	}
	printf("%.10lf\n", answer);
}