program ex(f1,f2);
 type
  zn=array [1..150,1..150] of longint;
  ka=array [1..150] of longint;
  fa=array [1..150] of longint;
  pa=array [1..150,1..150] of boolean;
  la=array [1..100000] of longint;
 var
  f1,f2:text;
  i,j,k,p,m,n,l,x,y,ans,now,tot:longint;
  flag:pa;
  pj:real;
  a:zn;
  d:ka;
  c:fa;
  f:la;
 procedure qsort(l,r:longint);
  var
   i,j,x,y:longint;
  begin
   i:=l; j:=r;
   x:=f[(l+r) div 2];
   repeat
    while f[i]<x do inc(i);
    while f[j]>x do dec(j);
    if i<=j then
     begin
      y:=f[i];
      f[i]:=f[j];
      f[j]:=y;
      inc(i);
      dec(j);
     end;
   until i>j;
   if l<j then qsort(l,j);
   if r>i then qsort(i,r);
  end;



 begin
  ans:=maxlongint;
  assign(f1,'hole.in'); reset(f1);
  assign(f2,'hole.out'); rewrite(f2);
  readln(f1,n,m);
  for i:=1 to n do
   for j:=1 to n do
    flag[i,j]:=true;
  for i:=1 to n do
   for j:=1 to n do
    a[i,j]:=99999;
  for i:=1 to n-1 do
   begin
    readln(f1,x,y,l);
    a[x,y]:=l;
    a[y,x]:=l;
   end;

  for k:=1 to n do
   for i:=1 to n do
    for j:=1 to n do
     if a[i,j]>(a[i,k]+a[k,j]) then a[i,j]:=a[i,k]+a[k,j];
  now:=0;
  for i:=1 to n do
   for j:=i+1 to n do
    if i<>j then
    begin inc(now); f[now]:=a[i,j]; end;
  qsort(1,now);
  for i:=1 to m do tot:=tot+f[i];
  pj:=tot/m;
  writeln(f2,pj:0:2);
  close(f1); close(f2);
 end.