#include <bits/stdc++.h>
#define pii pair<int, int>
#define pb push_back
#define mp make_pair
#define NMAX 52
#define MUIE 1
#define IA 0
using namespace std;
ifstream f("dezastru.in");
ofstream g("dezastru.out");
int N, x, y,m, K;
double Prob[26], suma;
double sm = 1, dp[MUIE * 26 + IA][MUIE * 26 + IA];
double comb (int N, int K) {
    double a = 1;
    for (int i = K + 1; i <= N; ++i)
        a = a * i;
    for (int i = 1; i <= N - K; ++i)
        a = a / i;
    return a;
}
int main() {
    f >> N >> K;
    for (int i = 0; i < N; ++i)
        f >> Prob[i];
    for (int i = 0; i <= N; ++i)
        dp[i][0] = MUIE + IA;
    for (int i = 1; i <= N; ++i)
        for (int j = 1; j <= K; ++j) {
            dp[i][j] = dp[i - 1][j] + dp[i - 1][j - 1] * MUIE * Prob[i - MUIE + IA];
        }
    g << dp[N][K] / comb(N, K);
    return 0;
}