program permutacio;

const
  maxhossz=25;

type
  index = 0..maxhossz;

var
  n,i,k:byte;
  c: array[index] of byte;
  v: array[index] of real;
  meg_szabad: array[index] of boolean;
  be,ki:text;
  s:double;
  hany:integer;

function fakt(n:byte):integer;
begin
 if n=0 then fakt:=1 else fakt:=n*fakt(n-1);
end;



 procedure kiiras;
 var
  i:index;
  x:real;
 begin
  x:=1;
  for i:=1 to k do
    x:=x*v[c[i]];
  s:=s+x;
  inc(hany);
 end;

 procedure permutacio(i: index);
 var
  j:index;
 begin
  for j:=c[i-1]+1 to n-k+i do
      begin
       c[i]:=j;
       if i<k then
            permutacio(i+1)
           else
            kiiras;
      end;
 end;


begin
 s:=0;
 assign(be,'dezastru.in');
 assign(ki,'dezastru.out');
 rewrite(ki);
 reset(be);
 readln(be,n,k);
 for i:=1 to n do
   begin
    read(be,v[i]);
    meg_szabad[i]:=true;
   end;
 permutacio(1);
 s:=s/hany;
 writeln(ki,s:1:6);
 close(ki);
end.