Imagine a scenario where an adventurous little robot, named Robi, has a mission to traverse a grid with m rows and n columns and reach a treasure box placed at $grid[m-1][n-1]$. Robi starts the journey from its home in the top-left corner of the grid, $grid[0][0]$. However, it faces a constraint that at any given time, it can only move in either of the two directions: downward or to the right.

Given the two integers, m and n, return the total number of unique paths that Robi can take from the grid's top-left corner to reach the bottom-right corner of the grid.

Constraints:

• 1 $\leq$ m, n $\leq$ 100