#include<fstream.h>
#include<math.h>
#include<stdio.h>
#include<iomanip.h>

int n, m;
float v[500], a[1000];

void citire()
{
  ifstream in("dezastru.in");
  in>>n>>m;
  for (int i=1; i<=n; i++)
    in>>v[i];
  in.close();
}



void calcul()
{
  int nr=1, i, j;
  float rez;
  for (i=1; i<=n; i++) nr*=i;
 /* for (i=1; i<=m; i++) nr2*=i;
  nr3=nr/nr2;*/
  int k=1;
  for (i=1; i<n; i++)
    for (j=i+1; j<=n; j++)
      a[k++]=v[i]*v[j];
  for (i=1; i<k; i++)
    rez+=a[i];
  rez=(rez*m)/nr;
  FILE *out=fopen("dezastru.out","w");
  fprintf(out,"%5.6f",rez);
  fclose(out);
}

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