#include <bits/stdc++.h>
#define pii pair<int, int>
#define pb push_back
#define mp make_pair
#define NMAX 52
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int N, x, y,m, K, cnt, st[26];
double Prob[26], suma;
double sm = 1;
void bkt (int k) {
    if (k == K + 1) {
        ++cnt;
        suma += sm;
        return;
    }
    for (int i = st[k - 1] + 1; i < N; ++i) { // sunt 5 milioane de combinari in pula mea de ce nu vrei sa intri in pizda matii
        st[k] = i;
        sm = sm * Prob[st[k]];
        bkt(k + 1);
        sm = sm / Prob[st[k]];
    }

}
int main() {
    f >> N >> K;
    for (int i = 0; i < N; ++i)
        f >> Prob[i];
    st[0] = -1;
    bkt (1);
    g << suma / cnt;
    return 0;
}