#include<stdio.h>

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

void afis() {
    double t = 1;
    for(int i = 1; i <= m; i++)
          t *= a[sol[i]];
    s += t;
    nr++;
}

void back(int k)
{
    if(k > m) {
        afis();
        return;
    }
    for(int i = sol[k-1] + 1; i <= n - m + k; i++) {
         sol[k] = i;
         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;
}