/*#include<stdio.h>

int n,m;
long long nr = 0;
double a[26], s;

void comb() {
    long long pp = 1, p2[25], n2 = 1 << n;
    for(int j = 0; j < n; j++, pp <<= 1)
        p2[j] = pp;
    for (long long  i = 0; i < n2; i++) {
        int nr1 = 0;
        double t = 1;
        for (int j = 0; j < n; j++) {
            if (i & p2[j])
                nr1++,t *= a[j];
        }
        if (nr1 == m)
            s += t, nr++;
    }
}

void citire() {
    freopen("dezastru.in","r",stdin);
    scanf("%d %d", &n,&m);
    for(int i = 0; i < n; i++)
        scanf("%lf", &a[i]);
}


int main() {
    citire();
    comb();
    freopen("dezastru.out","w",stdout);
    printf("%lf", s / nr);
    return 0;
}
*/
    #include<stdio.h>

    int n,m,sol[26];
    long long nr = 0;
    double a[26], s, p;

    void back(int k) {
       for(int i = sol[k-1] + 1; i <= n - m + k; i++) {
            sol[k] = i;
            if(k == m) {
                p = 1;
                for(int v = 1; v <= m; v++)
                    p *= a[sol[v]];
                s += p;
                nr++;

            }
            else
                back(k+1);
       }
    }

    void citire() {
       freopen("dezastru.in","r",stdin);
       scanf("%d %d", &n,&m);
       for(int i = 1; i <= n; i++)
           scanf("%lf", &a[i]);
    }


    int main() {
       citire();
       back(1);
       freopen("dezastru.out","w",stdout);
       printf("%lf", s / nr);
       return 0;
    }