const
  lim=25;
var
  dez:array[1..lim] of real;
  a,done:array[0..lim] of longint;
  cate,i,j,n,k:longint;
  t,sol:real;

{procedure calculeaza;
var
  i:longint;
begin
  t:=1;
  for i:=1 to k do
    t:=t*dez[a[i]];
  sol:=sol+t;
  inc(cate);
end;}

procedure doit(x:longint);
var
  i:longint;
  {r:real;}
begin
  if (x>k) then
  begin
    {calculeaza;}
    sol:=sol+t;
    inc(cate);
    exit;
  end;
  for i:=a[x-1]+1 to n-k+x do
    if done[i]=0 then
    begin
      a[x]:=i;
      done[i]:=1;
      {r:=t;}
      t:=t*dez[i];
      doit(x+1);
      t:=t/dez[i];
      done[i]:=0;
      a[x]:=0;
    end;
end;

begin
  assign(input,'dezastru.in');
  reset(input);
  readln(n,k);
  for i:=1 to n do read(dez[i]);
  close(input);
  t:=1;
  doit(1);
  sol:=sol/cate;
  assign(output,'dezastru.out');
  rewrite(output);
  writeln(sol:0:6);
  close(output);
end.