#include <fstream>
#include <iomanip>
#define NMAX 30

using namespace std;

ifstream fin ("dezastru.in");
ofstream fout ("dezastru.out");

int n, m, use[NMAX], currArrangement[NMAX], noAttacks;
double v[NMAX], localChance, globalChance;

void arrange (int k)
{
    int i;

    if (k == m+1) {
        noAttacks++;
        localChance = 1;

        for (i = 1; i <= m; i++) localChance *= v[currArrangement[i]];
        globalChance += localChance;

    }
    else {
        for (i = 1; i <= n; i++)
            if (!use[i]) {
                currArrangement[k] = i;
                use[i] = 1;
                arrange (k+1);
                use[i] = 0;
            }
    }
}

int main()
{
    fin >> n >> m;
    for (int i = 1; i <= n; i++) fin >> v[i];

    arrange (1);
    fout << fixed << setprecision(6) << globalChance / noAttacks << "\n";
    return 0;
}