// Problem: A. 地雷爆炸
// Contest: undefined - NOIP2024训练赛 02
// URL: http://119.28.3.174/contest/1154/problem/2053
// Memory Limit: 1024 MB
// Time Limit: 1000 ms
// 
// Powered by CP Editor (https://cpeditor.org)

#include<bits/stdc++.h>
//#define int long long
#define fi first
#define se second
#define PII pair<int, int>

using namespace std;

const int N = 1e5 + 10, M = 1e6 + 10, mod = 1e9 + 7, INF = 0x3f3f3f3f;
const double eps = 5e-5;

int n, m, q, ans;
double s[N], dp[305][306];

void dfs(int id, int now, int nd, double f, int sum)
{
	if(f < eps) return;
	if(now > nd)
	{
		if(sum <= 300) dp[id][sum] += f;
		return;
	}
	for(int i = 0; i < n; i ++) dfs(id, now + 1, nd, f * s[i], sum + i - 1);
}

signed main()
{
	// ios::sync_with_stdio(0), cin.tie(0), cout.tie(0);
	// cin >> n >> m;
	scanf("%d %d", &n, &m);
	for(int i = 0; i < n; i ++) scanf("%lf", &s[i]);
	// for(int i = 0; i < n; i ++) cout << s[i] << ' ';
	dp[0][1] = 1.0;
	for(int i = 1; i <= m; i ++)
	{
		dp[i][0] = dp[i - 1][0];
		for(int j = 1; j <= 300; j ++) dfs(i, 1, j, dp[i - 1][j], j); 
		// cout << dp[i][0] << '\n';
	}
	printf("%.4lf", dp[m][0]);
	return 0;
}