program hole;
type
  dian=record
    first,vdata:longint;
  end;
  bian=record
    v,next,w:longint;
  end;
  dptree=record
    left,right,lw,rw,vdata:longint;
  end;

var
  list:array[0..1500] of dian;
  map:array[0..3000] of bian;
  tree:array[0..1500] of dptree;
  f:array[0..1500,0..1000] of longint;
  b:array[0..1500] of boolean;
  n,m,i,j,r1,r2,r3,ans:longint;

function max(x,y:longint):longint;
begin
  if x>y
    then max:=x
    else max:=y;
end;

function min(x,y:longint):longint;
begin
  if x<y
    then min:=x
    else min:=y;
end;

procedure maketree(v:longint);
var
  i:longint;
begin
  tree[v].vdata:=1;
  i:=list[v].first;
  while i<>0 do
  begin
    if b[map[i].v]=false then
    begin
      if tree[v].left=0 then
      begin
        tree[v].left:=map[i].v;
        tree[v].lw:=map[i].w;
        b[map[i].v]:=true;
        maketree(map[i].v);
        tree[v].vdata:=tree[v].vdata+tree[tree[v].left].vdata;
      end
      else
      begin
        tree[v].right:=map[i].v;
        tree[v].rw:=map[i].w;
        b[map[i].v]:=true;
        maketree(map[i].v);
        tree[v].vdata:=tree[v].vdata+tree[tree[v].right].vdata;
      end;
    end;
    i:=map[i].next;
  end;
end;

function dp(i,j:longint):longint;
var
  i1,num1:longint;
begin
  if (tree[i].vdata=1) or (j=0) then exit(0);
  if f[i,j]<>-1 then exit(f[i,j]);

  if tree[i].right=0 then
  begin
    f[i,j]:=dp(tree[i].left,j-1)+(j-1)*(m-(j-1))*tree[i].lw;
  end
  else
  begin
    f[i,j]:=maxlongint;
    for i1:=max(0,j-tree[tree[i].right].vdata-1) to min(j-1,tree[tree[i].left].vdata) do
    begin
      if i1>tree[tree[i].left].vdata then continue;
      if j-1-i1>tree[tree[i].right].vdata then continue;
      num1:=dp(tree[i].left,i1)+i1*(m-i1)*tree[i].lw
            +dp(tree[i].right,j-1-i1)+(j-1-i1)*(m-(j-1-i1))*tree[i].rw;
      if num1<f[i,j] then f[i,j]:=num1;
    end;
  end;
  dp:=f[i,j];
end;

begin
  assign(input,'hole.in');
  reset(input);
  assign(output,'hole.out');
  rewrite(output);

  readln(n,m);
  for i:=0 to n do
    for j:=0 to m do
      f[i,j]:=-1;
  fillchar(list,sizeof(list),0);
  for i:=1 to n-1 do
  begin
    readln(r1,r2,r3);

    map[i*2-1].v:=r2;
    map[i*2-1].w:=r3;
    map[i*2-1].next:=list[r1].first;
    list[r1].first:=i*2-1;
    inc(list[r1].vdata);

    map[i*2].v:=r1;
    map[i*2].w:=r3;
    map[i*2].next:=list[r2].first;
    list[r2].first:=i*2;
    inc(list[r2].vdata);
  end;

  fillchar(b,sizeof(b),false);
  b[1]:=true;
  maketree(1);

  fillchar(f,sizeof(f),$FF);
  ans:=dp(1,m);
  writeln(ans/(m*(m-1)/2):0:2);

  close(input);
  close(output)
end.