Hi, could someone help me with this problem - suppose I have s identical balls that I want to put in n identical boxes - in how many different way can I do this?

For example: s=3, n=3:

|ooo|||

|oo|o||

|o|o|o|

So the answer is 3.

s=6, n=2:

|oooooo||

|ooooo|o|

|oooo|oo|

|ooo|ooo|

So the answer is 4.

It's easy to do it ad hoc, but the general principle escapes me.

Thanks!