#include <inttypes.h>
#include <stdio.h>
#include <string.h>

#define SIZE 25

static double sum, p[SIZE];
static int np, sol[SIZE+1];

static void comb(int n, int k)
{
    int pos, filled_pos;
    double prod, prods[SIZE];

    pos = 1;
    prod = 1.0;
    while (pos > 0) {
        filled_pos = 0;
        while (sol[pos] < n - k + pos && !filled_pos) {
            sol[pos]++;
            filled_pos = 1;
        }

        if (filled_pos) {
            prods[pos] = prod;
            prod *= p[sol[pos] - 1];

            if (pos == k) {
                sum += prod;
                np++;
                prod = prods[pos];
            } else {
                pos++;
                sol[pos] = sol[pos - 1];
            }
        } else {
            pos--;
            prod = prods[pos];
        }
    }
}

int main(void)
{
    int i, n, k;

    freopen("dezastru.in", "r", stdin);
    freopen("dezastru.out", "w", stdout);

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

    for (i = 0; i < n; i++) {
        scanf("%lf", &p[i]);
    }

    comb(n, k);

    printf("%lf", sum / np);

    return 0;
}