#include<cstdio>
using namespace std;
FILE *f=fopen("dezastru.in","r");
FILE *g=fopen("dezastru.out","w");
int n, k, st[30], numar;
float v[30], s, x[10000000];
int valid(int p)
{
    int i;
    for(i=1;i<p;i++)if(st[i]==st[p])return 0;
    return 1;
}
int solutie(int p)
{
    if(p==n)return 1;
    return 0;
}
void afis(int p)
{
    int i;
    float nr=1;
    for(i=1;i<=k;i++)nr*=v[st[i]];
    x[++numar]=nr;
}
void bkt(int p)
{
    int i;
    for(i=1;i<=n;i++)
    {
        st[p]=i;
        if(valid(p))
        {
            if(solutie(p))afis(p);
            else bkt(p+1);
        }
    }
}
int main()
{
    fscanf(f,"%d %d",&n,&k);
    int i;
    for(i=1;i<=n;i++)fscanf(f,"%f",&v[i]);
    bkt(1);
    for(i=1;i<=numar;i++)s+=(float)(x[i]/numar);
    fprintf(g,"%6f",s);
    fclose(f);
    fclose(g);
    return 0;
}