#include <fstream>
#include <string>
#include <stdio.h>
#include <vector>
#include <algorithm>
#include <math.h>
#include <set>
#include <map>
#include <string.h>
#include <queue>
#include <stack>

#define INF 0x3f3f3f3f

using namespace std;

#ifdef DEBUG
string name = "data";
#else
string name = "dezastru";
#endif

ifstream fin(name + ".in");
ofstream fout(name + ".out");

int n,k;

double p[50];

double best = 0;
int cnt = 0;

void back(int i, int s, double sum) {
    if (s == k) {
        best += sum;
        cnt++;
        return;
    }
    
    for (int j = i; j <=n; ++j) {
        back(j + 1, s + 1, sum * p[j]);
    }
}

int main() {
    
    fin >> n >> k;
    
    for (int i = 1; i <= n; ++i) {
        fin >> p[i];
    }
    
    back(1, 0, 1);
    
    fout << best / cnt;
    
    return 0;
}