var n,k,nr,i:longint;
    rs:real;
    a:array[0..30] of byte;
    b:array[0..30] of real;

  procedure back(lvl:longint);
   var i:longint;
       aux:real;
    begin
     if lvl=k then begin
                    inc(nr); aux:=1;
                    for i:=1 to k do
                     aux:=aux*b[a[i]];
                    rs:=rs+aux;
                   end
            else for i:=a[lvl]+1 to n do
                  begin
                   a[lvl+1]:=i;
                   back(lvl+1);
                  end;
    end;

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

   readln(n,k); rs:=0; nr:=0;
    for i:=1 to n do
     read(b[i]);

     back(0);

    rs:=rs/nr;
   writeln(rs:0:6);


 close(input);
 close(output);
{Totusi este trist in lume}
end.