There is a robot on an `m x n` grid. The robot is initially located at the **top-left corner** (`grid[0][0]`) and wants to reach the **bottom-right corner** (`grid[m - 1][n - 1]`). The robot can only move either **down** or **right** at any point.
Given the two integers `m` and `n`, return the number of possible unique paths the robot can take.
**Example 1:**
```
Input: m = 3, n = 7
Output: 28
```
**Example 2:**
```
Input: m = 3, n = 2
Output: 3
Explanation: Right→Down→Down, Down→Right→Down, Down→Down→Right
```