## 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
def sum_mul(n, m):
    return sum(range(n, m, n)) if n > 0 < m else 'INVALID'
def sum_mul(n, m):
    return sum(range(n, m, n)) if n > 0 < m else 'INVALID'