#include <stdio.h>
#include <stdlib.h>
int v[30],f[30];
long long nr;
double x[30],sum=0;
long long fact(int k)
{
    int i;
    long long s=1;
    for(i=2; i<=k; i++)
        s=s*(long long)i;
    return s;
}
void bkt(int n,int p,int k)
{
    int i=1;
    double prod=1;
    if(p>k){
        prod=x[v[1]];
        nr+=(fact(k));
        for(i=2; i<=k; i++)
            prod*=x[v[i]];
        sum+=(prod*(double)(fact(k)));
    }
    else
        for(v[p]=v[p-1]+1; v[p]<=n; v[p]++)
            if(f[v[p]]==0)
            {
                f[v[p]]=1;
                bkt(n,p+1,k);
                f[v[p]]=0;
            }
}
int main()
{
    int n,k,i;
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    scanf("%d%d",&n,&k);
    f[n+1]=1;
    for(i=1; i<=n; i++)
        scanf("%lf",&x[i]);
    bkt(n,1,k);
    printf("%lf\n",sum/(double)nr);

    return 0;
}