#include <cstdio>
#define FIN "dezastru.in"
#define FOUT "dezastru.out"
#define N 30

int n, k, nr;
double v[N];
double rez;

void read()
{
    int i;

    freopen(FIN, "r", stdin);
    freopen(FOUT, "w", stdout);

    scanf("%d %d", &n, &k);

    for (i = 1; i <= n; ++i)
        scanf("%lf", &v[i]);
}

void write()
{
    printf("%.6lf ", rez / nr);
}

void solve(int u, int l, double p)
{
    int i;

    if (l == k)
    {
        rez += p;
        ++nr;
        return;
    }

    for (i = u + 1; i <= n + l - k + 1; ++i)
        solve(i, l + 1, p * v[i]);

}

int main()
{
    int i;

    read();

    for (i = 1; i <= n - k + 1; ++i)
        solve(i, 1, v[i]);

    if (k == 0)     rez = 100;

    write();
}