Unique Paths in a Grid

You need to find all possible paths from the top-left corner (0,0) to the bottom-right corner (m-1, n-1) in a 2D grid. You can only move right or down. Write a function num_paths(m, n), where m and n represent the number of rows and columns of the grid, respectively. The function should return an integer indicating the number of all possible paths. Assume that all paths in the grid are unobstructed. Example Input: m = 3, n = 7 Output: 28 Constraints 1 <= m, n <= 100 The result is guaranteed to fit in a 32-bit integer. Example Input 3 7