Solve the programming task below in a Python markdown code block.
## Your Job

  Find the sum of all multiples of `n` below `m` 
  
  
## Keep in Mind

  * `n` and `m` are natural numbers (positive integers)
  * `m` is **excluded** from the multiples
  
  
## Examples
Also feel free to reuse/extend the following starter code:
```python
def sum_mul(n, m):
```