#include <stdio.h>
using namespace std;
FILE*f=fopen("dezastru.in","r");
FILE*g=fopen("dezastru.out","w");
int n,k,p,as,ev,st[30],l;
float v[30],x[1001];
int succ(int k)
{
    if (st[k]<n)
    {
        st[k]++;
        return 1;
    }
    return 0;
}
int valid(int k)
{
    for (int i=1;i<k;i++)
    {
        if (st[i]==st[k]) return 0;
    }
    return 1;
}
int sol(int k)
{
    if (k==p) return 1;
    return 0;
}
void citire(int n)
{
    for (int i=1;i<=n;i++)
    {
        fscanf(f,"%f",&v[i]);
    }
}
double prod(int k)
{
    double p=1;
    for (int i=1;i<=k;i++)
    {
        p*=v[st[i]];
    }
    return p;
}
double medie(int k)
{
    double s;
    for (int i=1;i<=l;i++)
    {
        s+=x[i];
    }
    return (s/l);
}
int main()
{
    fscanf(f,"%d%d",&n,&p);
    k=1; st[k]=0;
    citire(n);
    while (k>0)
    {
        do
        {
            as=succ(k);
            if (as) ev=valid(k);
        }while(as && !ev);
        if (as)
        {
            if (sol(k))
            {
                l++;
                x[l]=prod(k);
            }
            else
            {
                k++;
                st[k]=0;
            }
        }
        else k--;
    }
    fprintf(g,"%f",medie(l));
    return 0;
}