#include<fstream>
#include<iomanip>

using namespace std;
typedef int var;

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

#define MAXN 26

double P[MAXN];
double global_prob;
var nr_perm;


void gen(var &n, var &k, var pas, var taken, double prob) {
    if(taken >= k) {
        global_prob += prob;
        nr_perm ++;
        return;
    }
    if(pas - 1 + k - taken > n) {
        return;
    }

    gen(n, k, pas+1, taken, prob);
    gen(n, k, pas+1, taken+1, prob * P[pas]);
}


int main() {
    var n, k;
    fin>>n>>k;
    for(var i=1; i<=n; i++) {
        fin>>P[i];
    }
    gen(n, k, 1, 0, 1.0);

    fout<<setprecision(7)<<fixed<<global_prob / nr_perm;

    return 0;
}