How would we solve the knapsack problem if we now have to fix the number of items in the knapsack by a constant $L$? This is the same problem (max weight of $W$, every item have a value $v$ and weight $w$), but you must add exactly $L$ item(s) to the knapsack (and obviously need to optimize the total value of the knapsack).
Every way I've thought of implementing this so far (dynamic programming, brute force) has resulted in either failure, or lifetime-of-universe level computation times. Any help or ideas are appreciated.
Edit: I am looking for pseudo-polynomial time solutions