#include <fstream.h>

float a[26];

int main ()
{

long double fact=1;
float prod=1,fin=0;
int k,i,n,j,l;

ifstream f("dezastru.in");
ofstream g("dezastru.out");

f>>n;
f>>k;


for (i=1;i<=n;i++)
{
fact=fact*i;
}

for (i=0;i<n;i++)
{
f>>a[i];
}
for (i=0;i<n-k;i++)
{
	for (j=i;j<=n-k;j++)
	{
		prod=a[j];
		for (l=1;l<=k;l++)
		{
		prod=a[j];
		prod=prod*a[j+l];
		fin=fin+2*(prod/fact);
		}
	}
}

g<<fin;

g.close ();
f.close ();
return 0;
}