var a:array[0..100000] of 0..1;
f,g:text;
i,n,m,x:longint;
r:longint;
ok:boolean;
begin
assign(f,'grigo.in');
reset(f);
assign(g,'grigo.out');
rewrite(g);
  read(f,n,m);
   for i:=1 to m do begin
                    read(f,x);
                    a[x-1]:=1;
                    end;
   x:=1;
   for i:=1 to n-1 do
        if a[i]=0 then x:=(x*i);
   writeln(g,r);
   close(g);
end.