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

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

void read()
{
    freopen(FIN, "r", stdin);
    freopen(FOUT, "w", stdout);
    scanf("%d %d", &n, &k);
    for (int i = 1; i <= n; ++i)
        scanf("%f", &v[i]);
}

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

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

    if (l == k)
    {
        rez += p;
        ++nr;
        return;
    }
    for (i = u + 1; i <= n; ++i)
        solve(i, l + 1, p * v[i]);
    return;
}

int main()
{
    int i;

    read();

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

    if (k == 0)     rez = 100;

    write();
}