var p,s:extended;
a:array[0..25] of extended;
v:array[0..25] of integer;
i,n,m,nr,k:longint;
f,g:text;

begin
assign(f,'dezastru.in');
assign(g,'dezastru.out');
reset(f);
rewrite(g);
read(f,n,m);
for k:=1 to n do
	read(f,a[k]);
k:=1;
a[0]:=1;
v[1]:=0;
p:=1;
while k<>0 do
	if v[k]<n then  begin
   	inc(v[k]);
      p:=p/a[v[k]-1]*a[v[k]];
      if k=m then begin
      	s:=s+p;
         inc(nr);
         end
      else begin
      	inc(k);
			v[k]:=v[k-1];
      	p:=p*a[v[k]];
         end;
      end
   else begin
   	p:=p/a[v[k]];
   	k:=k-1;
      end;
writeln(g,s/nr:0:6);
close(f);
close(g);
end.