#include<fstream.h>
#include<iomanip.h>
#define InFile "dezastru.in"
#define OutFile "dezastru.out"
#define Max 2000
float a[Max];
int n,k;
void citire()
{ ifstream f(InFile);
  f>>n>>k;
  int i;
  for(i=1;i<=n;i++)
     f>>a[i];
  f.close();
}
void prelucrare()
{ float t1,t2,t3;
  int i,j;
  t1=t2=t3=1;
  float s=0;
  for(i=2;i<=n;i++)
      t1=t1*i;
  for(i=2;i<=k;i++)
      t2=t2*i;
  for(i=2;i<=n-k;i++)
      t3=t3*i;
  t1=2*t1/(t2*t3);
  for(i=1;i<=n;i++)
     for(j=i+1;j<=n;j++)
	 s=s+2*a[i]*a[j];
  s=s/t1;
  ofstream g(OutFile);
  g<<setprecision(6)<<s;
  g.close();

}
int main()
{ citire();
  prelucrare();
  return 0;
}