#include<stdio.h>
double a[30];
int nr,n,m;
double sum;
void back (int k,int start,double p)
{
    int i;
    if(k==m+1)
    {
        sum+=p;
        ++nr;
    }
    else
        for(i=start;i<=n-m+k;++i)
            back(k+1,i+1,p*a[i]);
}
int main ()
{
    freopen("dezastru.in","r",stdin);
    freopen("dezastru.out","w",stdout);
    int i;
    scanf("%d%d",&n,&m);
    for(i=1;i<=n;++i)
        scanf("%lf",&a[i]);
    back (1,1,1);
    printf("%lf",sum/nr);
    return 0;
}