Hi,Codeforces! I need your help!

There is a kind of integer sequence of length (n+k). (k<=n)

Constraints follow:

1. For every element in sequence, its value is either -1 or a positive integer.

2. The number of -1's occurrences is n and the number of positive integers's occurrences is k.

3. The sum of all positive integers equals n.

4. Any prefix sum of the sequence is non-negative.

Now you must calculate the number of different sequences satisfying the constraints above.

Is there a general formula for this answer? Most thanks.