A robot is placed at the top-left corner of an m x n grid and needs to reach the bottom-right corner. The robot can only move right or down at each step. Return the total number of unique paths from the top-left to the bottom-right corner.
Return null if either m or n is null.
Example 1:
Input: m = 3, n = 7
Output: 28
Explanation: There are 28 unique paths in a 3x7 grid moving only right or down.
Example 2:
Input: m = 3, n = 2
Output: 3
Explanation: Three unique paths exist in a 3x2 grid.
Example 3:
Input: m = 1, n = 1
Output: 1
Explanation: The robot is already at the destination, so there is exactly 1 path.
Example 4:
Input: m = 1, n = 5
Output: 1
Explanation: In a single-row grid, the robot can only move right. There is only 1 path.
Example 5:
Input: m = null, n = 7
Output: null
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