input: Write a python function to find the number of unique paths in an m x n grid.
  assert numPaths(3, 7) == 28
  assert numPaths(3, 2) == 3
  assert numPaths(7, 3) == 28
  assert numPaths(3, 3) == 6
output: def numPaths(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]