How many ways can you walk from A to B such that you always stay below the diagonal? (more details included)?

Question

There is a square grid. In the lower left corner is A and in the upper right is B. A diagonal is drawn from A to B and you are not allowed to go above it while travelling from A to B. You can only move right and up. In how many ways in an n x n grid can you travel from A to B? Prove your answer.

(I know that without the diagonal, there are (2n) C (n) ways.)

Answer 1

C(2n, n) / (n+1)