Program dezastru;
 var a:array [0..25] of byte;
     b:array [0..25] of double;
     sum:double;
     nr:longint;
     n,k,i:byte;
     fi,fo:text;
procedure back(l:byte);
var i:byte;
     s:double;
begin
 if l=k then begin
               s:=1;
              for i:=1 to k do s:=s*b[a[i]];
               sum:=sum+s; inc(nr);
              end
  else for i:=a[l]+1 to n do begin
                           a[l+1]:=i;
                           back(l+1);
                           end;
end;
begin
 assign(fi,'dezastru.in');
  assign(fo,'dezastru.out');
 reset(fi); rewrite(fo);
 readln(fi,n,k);
  for i:=1 to n do read(fi,b[i]);
   back(0);
    write(fo,sum/nr:0:6);
 close(fo);
end.