program ProjectC;

{$APPTYPE CONSOLE}

uses
  SysUtils;

type
   Arr = array of Integer;
var
   ArrA, ArrB: Arr;
   fact, N: Integer;
   i, k: Integer;

begin
   readln(N);
   k:=-1;
   SetLength(ArrA, N);
   for i := 0 to n - 1 do
      readln(ArrA[i]);
   SetLength(ArrB, N);
   for i := 0 to n - 1 do
      if ArrA[i] <= N then
      begin
         inc(k);
         ArrB[k] := ArrA[i];
      end;
   fact := 1;
   if k <> -1 then
   begin
      for i := 2 to k + 1 do
         fact := fact * i;
      fact := fact div 2;
   end
   else
      fact := 1;
   writeln(fact);
   readln;
end.