#include<fstream>

using namespace std;

int n,k,p,c=1,l;
float a[100],s=1,b[100],s1;
int main()
{
    int i,j,cont=0;
    ifstream fin("dezastru.in");
    ofstream fout("dezastru.out");
    
    fin>>n>>k;
    for(i = 1; i <= n; i++)
    fin>>a[i];
    if(k == n)
    {
    for(i = 1; i <= n; i++)
    s*=a[i];
    p=k;
    fout<<n*s/p;
    } 
    else
    {   
for( i = 2; i <= n; i++)
{
    for( j = 1; j <= k-1; j++)
        if( i != j)
        s*=a[j];
        b[l++]=a[i]*s;
        s=1;

} 
while( k <= n)
{
    for( i = c; i <= k; i++)
        s*= a[i];
        c++;
        k++;
        for( j = 0; j <= l; j++)
        {
        if(b[j]!=s)
        cont = 1;
        if(b[j] == s)
        {cont=0;break;}
        if(cont == 1)
        b[l++] = s;
        }       
        s=1;
}      
       for(i = 0; i < l-1; i++)
         s1+=2*b[i]/(2*(l-1));
         fout<<s1;
     }       
         fin.close();
          fout.close();
          
    return 0;
    
}