input: Write a python function to return the number of possible paths from the top-left to the bottom-right in an mxn matrix.
  assert num_of_paths(3, 3) == 6
  assert num_of_paths(2, 2) == 2
  assert num_of_paths(4, 4) == 20
output: def num_of_paths(m, n):
      dp = [[1] * n for _ in range(m)]
  
      for i in range(1, m):
          for j in range(1, n):
              dp[i][j] = dp[i-1][j] + dp[i][j-1]
  
      return dp[m-1][n-1]