#include <bits/stdc++.h>
#define MOD 104659
using namespace std;
///aproapeperm
ifstream fin("dezastru.in");
ofstream fout("dezastru.out");

int N, K, cnt;
double a[30], sum;
int st[30], top;

void Back(int top)
{
    if (top == K + 1)
    {
        double produs = 1;
        cnt++;
        for (int i = 1; i < top; i++)
            produs *= a[st[i]];
        sum += produs;
    }
    else for (int i = st[top - 1] + 1; i <= N; i++)
         {
             st[top] = i;
             Back(top + 1);
         }
}

int main()
{
    fin >> N >> K;
    for (int i = 1; i <= N; i++)
        fin >> a[i];
    Back(1);
    fout << fixed << setprecision(6) << sum / cnt << "\n";
    return 0;
}