There is a robot on an `m x n` grid. The robot is initially located at the top-left corner (i.e., `grid[0][0]`). The robot tries to move to the bottom-right corner (i.e., `grid[m - 1][n - 1]`). The robot can only move either down or right at any point in time.

Given the two integers `m` and `n`, return the number of possible unique paths that the robot can take to reach the bottom-right corner.

The test cases are generated so that the answer will be less than or equal to `2 * 109`.

Example 1:

    Input: m = 3, n = 7
    Output: 28

Example 2:

    Input: m = 3, n = 2
    Output: 3
    Explanation: From the top-left corner, there are a total of 3 ways to reach the bottom-right corner:
    1. Right -> Down -> Down
    2. Down -> Down -> Right
    3. Down -> Right -> Down

Constraints:

- `1 <= m, n <= 100`