#include <fstream>
#include <iomanip>

using namespace std;

ifstream in("dezastru.in");
ofstream out("dezastru.out");

const int sizee = 25;
int n, k;
float prob[sizee + 2];

float dp[sizee + 2][sizee + 2];

float ank(int n, int k){
    ///cnk(n, k) = n! / (k! * (n - k)!);
    ///ank(n, k) = n! / (n - k)!

    float ank = 1;
    for(int i = n - k + 1; i <= n; i++)
        ank *= i;
    return ank;
}

int main(){
    in>>n>>k;
    for(int i = 1; i <= n; i++)
        in>>prob[i];

    dp[1][0] = 1;
    dp[1][1] = prob[1];

    for(int i = 2; i <= n; i++){
        dp[i][0] = 1;
        for(int j = 1; j <= min(i, k); j++)
            dp[i][j] = dp[i - 1][j] + dp[i - 1][j - 1] * prob[i];
    }

    out<<fixed<<setprecision(6)<<2 * dp[n][k] / ank(n, k)<<"\n";
    return 0;
}