var a:array[1..25] of real;
    b:array[0..25,0..25] of real;
    n,k:longint;
procedure citesc;
 var i:longint;
     f:text;
 begin
  assign(f,'dezastru.in'); reset(f);
  read(f,n,k);
  for i:=1 to n do
   read(f,a[i]);
  for i:=0 to n do begin
   b[0,i]:=0;
   b[i,0]:=1;
  end;
  close(f);
 end;
function aflu:real;
 var i,j:longint;
     q:longint;
 begin
  q:=1;
  if (n-k)>k then begin
   for i:=n-k to n do
    q:=q*i;
   for i:=1 to k do
    q:=q div i;
  end
  else begin
   for i:=k+1 to n do
    q:=q*i;
   for i:=1 to n-k do
    q:=q div i;
  end;
  aflu:=q;
 end;
procedure rezolvare;
 var i,j:longint;
     g:text;
     t,q:real;
 begin
  for i:=1 to n do
   for j:=1 to k do
    b[i,j]:=b[i-1,j]+b[i-1,j-1]*a[i];
  q:=aflu;
  assign(g,'dezastru.out'); rewrite(g);
  t:=(b[n,k])/q;
  writeln(g,t:0:6);
  close(g);
 end;
begin
 citesc;
 rezolvare;
end.