Well done to Johann, Ingo and all others who solved last week's riddle. I was surprised to find that my solution was not as good as theirs.
As i promised , this week i'll give a hard one. It took me a month to solve and i'll buy a uni bar guiness to the first person to solve it this week. Since it involves a calculation I don't want only the answer but also the general method for solving it without a computer. A mathematician friend of mine solved it in about 5 mins and said any decent mathematician should be able to solve it in less then 10.
The riddle: 2000!mod2003
where ! is factorial and mod is the modulo defined as the operation that finds the remainder of division of one number by another eg 7mod5=2 or 39mod3=0.

Hint:2003 is a prime number

If you know the easy (mathematician's solution) you don't get a beer.

Have fun.