## Unique Paths

A robot is located at the top-left corner of a m x n grid (marked 'Start' in the diagram below).

The robot can only move either down or right at any point in time. The robot is trying to reach the bottom-right corner of the grid (marked 'Finish' in the diagram below).

How many possible unique paths are there?

### Examples

**Example 1**

<p align="left">
  <img src="../../assets/robot_maze.png" alt="Robot Maze">
</p>

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

**Example 2**

```text
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
```

**Example 3**

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

**Example 4**

```text
Input: m = 3, n = 3
Output: 6
```

---

### Constraints

- `1 <= m, n <= 100`
- It's guaranteed that the answer will be less than or equal to `2 * 109`.