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#pragma GCC optimize("Ofast")
#include<bits/stdc++.h>
bool Mbg;
using namespace std;
#define vec vector
#define pb push_back
#define eb emplace_back
#define siz(vec) (int)((vec).size())
#define all(vec) (vec).begin(),(vec).end()
template<class T>
void operator +=(vec<T> &lhs,const T &rhs){lhs.push_back(rhs);}
#define pii pair<int,int>
#define x first
#define y second
#define mp make_pair
#define exc(expr) if(expr)continue;
#define stop(expr) if(expr)break;
#define ret(expr) if(expr)return;
#define deb(var) cerr<<#var<<'='<<(var)<<"; "
#define debl(var) cerr<<#var<<'='<<(var)<<";\n"
#define ins insert
#define era erase
#define lb lower_bound
#define ub upper_bound
// #define int long long
#define inf (long long)(1e18)
template<class T>
bool Min(T &x,const T &y){return x>y?x=y,1:0;}
template<class T>
bool Max(T &x,const T &y){return x<y?x=y,1:0;}
const int mod=998244353;
void Add(int &x,const int &y){x=x+y<mod?x+y:x+y-mod;}
void Dec(int &x,const int &y){x=x>=y?x-y:x-y+mod;}
int fpm(int x,int y){
    int ans=1;for(;y;y>>=1,x=1ll*x*x%mod)if(y&1)ans=1ll*x*ans%mod;return ans;
}
const int B=1500;

int n,k;
double p[50010],f[100010],g[100010];

void work(){
    cin>>n>>k;
    for(int i=1;i<=n;i++){
        cin>>p[i];
    }
    sort(p+1,p+n+1,greater<double>());

    f[n+1]=1;
    double ans=0;

    double expe=n+1;
    for(int i=1;i<=n;i++){

        expe+=2*p[i]-1;

        int L=max((int)expe-B,n+1-i),R=min((int)expe+B,n+1+i);

        for(int j=R;j>=L;j--){
            f[j]=f[j-1];
        }
        // if(f[L]>1e-12){
        //     cerr<<fixed<<setprecision(10)<<f[L]<<'\n';
        // }
        double prob=0;
        for(int j=L;j<=R;j++){
            f[j]=f[j]*p[i]+f[j+2]*(1-p[i]);

            // deb(j),debl(f[j]);
            if(j>=n+1+k){
                prob+=f[j];
            }
        }

        // if(p[i]<0.5)deb(i),debl(pr);
        // debl(p[i]);

        Max(ans,prob);
        // if(p[i]<0.5&&ans-pr>eps){
            // break;
        // }
    }

    cout<<fixed<<setprecision(7)<<ans<<'\n';
}
bool Med;
signed main(){
    ios::sync_with_stdio(0),
    cin.tie(0),cout.tie(0);
    int T=1;while(T--)work();
    // cerr<<"Time: "<<clock()<<" ms;\n";
    // cerr<<"Memory: "<<abs(&Med-&Mbg)/1024.0/1024.0<<" MiB.\n";
}
/*
- CONTINUE, NON-STOPPING, FOR THE FAITH
- START TYPING IF YOU DON'T KNOW WHAT TO DO
- STOP TYPING IF YOU DON'T KNOW WHAT YOU'RE DOING
*/