#include <bits/stdc++.h>
using namespace std;
int N, K;
vector<double> p;
double sumP = 0;
void bt(int i, int c, double pr) {
    if (c == K) { sumP += pr; return; }
    for (int j = i; j < N; ++j)
        bt(j+1, c+1, pr * p[j]);
}
int main(){
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    cin >> N >> K;
    p.resize(N);
    for(int i=0;i<N;i++) cin >> p[i];
    bt(0,0,1.0);
    double comb = 1;
    for(int i=1;i<=K;i++) comb = comb * (N - K + i) / i;
    cout << fixed << setprecision(6) << sumP/comb << "\n";
}