Hàm tính Giai thừa trong Pascal

Program giaiThua;
uses crt;
var n: longint;
Function Giai_Thua(n:longint):longint;
Var GT:Longint;
begin
     GT:=1;
     while n > 0 do
     begin
     GT:=GT * n;
     n:=n-1;
     end;
     Giai_thua:=GT;
end;
begin
    clrscr;
    write('Nhap n: '); readln(n);
    write(n,'!=',Giai_thua(n));
    readln
end.