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#include<cstdio>
#include<vector>
#include<algorithm>

int main()
{
    std::vector<long double> v;
    std::vector<long double> probs[2];

    int t,n;
    scanf("%d %d", &t, &n);
    long double temp;
    for(int i=0; i<t; i++)
    {
        scanf("%Lf", &temp);
        v.push_back(temp);
    }
    std::sort(v.begin(), v.end(), std::greater<long double>());

    probs[0] = {1.0-v[0], 0.0, v[0]};
    int cur,prev;
    long double failureProb;

    long double curMaxProb = 0;
    long double tempMaxProb;

    for(int i=1; i<t; i++)
    {
        cur=i%2;
        prev=(i+1)%2;

        failureProb=1.0-v[i];

        probs[cur] = {};
        probs[cur].push_back(probs[prev][0]*failureProb);
        probs[cur].push_back(0.0);

        for(int j=1; j<2*i; j++)
        {
            probs[cur].push_back(probs[prev][j-1]*v[i] + probs[prev][j+1]*failureProb);
        }

        probs[cur].push_back(0.0);
        probs[cur].push_back(probs[prev][2*i]*v[i]);

        
        if(i>=(n-1) && (i-n+1)%2==0)
        {
            tempMaxProb=0;
            for(int k=i+n+1; k<2*i+3; k++)
            {
                tempMaxProb += probs[cur][k];
            }
            if(curMaxProb<tempMaxProb) curMaxProb = tempMaxProb;
            else break;
        }
    }
    printf("%.10Lf", curMaxProb);

    return 0;
}