var c:array[..] of longint;

     x,n,i,j,s:longint;

     ans:int64;

 begin

   readln(n);

   if n= then

   begin

     readln(x);

     if x= then writeln()

     else writeln();

     halt;

   end;

   for i:= to n do

   begin

     read(x);

     if (x<) or (x>n) then

     begin

       writeln();

       halt;

     end;

     for j:= to x- do

       inc(c[j]);

     s:=s+x-;

   end;

   if s<>n- then

   begin

     writeln();

     halt;

   end;

   ans:=;

   for i:= to n- do

   begin

     ans:=ans*int64(i);

     for j:= to n do

     begin

       while (c[j]>) and (ans mod j=) do

       begin

         dec(c[j]);

         ans:=ans div j;

       end;

     end;

   end;

   writeln(ans);

 end.