Please solve the programming task below using a self-contained code snippet in a markdown code block.

Given an m x n matrix mat where every row is sorted in strictly increasing order, return the smallest common element in all rows.
If there is no common element, return -1.
 
Please complete the following python code precisely:
```python
class Solution:
    def smallestCommonElement(self, mat: List[List[int]]) -> int:
```