#include<stdio.h>
 
   int main()
{
      double  a[26], b[26], c[26], n, k, s=1, suma=0, p, d, i, j;

    freopen("dezastru.in", "r", stdin);
    freopen("dezastru.out", "w", stdout);

scanf("%lf %lf", &n, &k);


for(i=1;i<=n;i++)
{ scanf("%lf", &a[i]); b[i]=c[i]=i; }

  while(s<=n)
{   c[n]=b[1];
   for(i=1;i<n;i++)
   { c[i]=b[i+1]; }  
 
   p=1;

     for(j=1;j<=k;j++)
      { p=p*a[c[j]]; }

     suma=suma+p; 

   s++;

   for(i=1;i<=n;i++)
   {  b[i]=c[i]; }

}  
    

 d=(suma*2)/(n*k);

printf("%lf", d);

return 0;

}