#include<cstdio>
#include<algorithm>
using std::sort;
using std::max;
inline bool cmp(double x,double y){
	return x>y;
}
constexpr size_t MAX=(size_t)5e3;
int n,k;
double p[MAX+1],dp[MAX+1][(MAX<<1)+1],tmp,ans=0;
inline void init(){
	for(int i=0;i<=MAX;++i) for(int j=0;j<MAX<<1;++j) dp[i][j]=0;
	dp[0][MAX]=1;
	return;
}
int main(){
	init();
	scanf("%d%d",&n,&k);
	for(int i=1;i<=n;++i) scanf("%lf",p+i);
	sort(p+1,p+n+1,cmp);
	for(int i=1;i<=n;++i){
		tmp=0;
		for(int j=0;j<=MAX<<1;++j){
			if(j) dp[i][j]+=dp[i-1][j-1]*p[i];
			if(j<MAX<<1) dp[i][j]+=dp[i-1][j+1]*(1-p[i]);
			if(j>=MAX+k) tmp+=dp[i][j];
		}
		ans=max(ans,tmp);
	}
	printf("%.7lf",ans);
	return 0;
}