shower thoughts led me to this. correct me if I'm wrong

retirement_function.py

def retirement(s,y,n,r):
  """
  Considering:
    - You have 0 debt
    - You make y per year (yearly income)
    - You can save a fraction s per year
    - You can invest at a r-1 yearly interest
    
  This function returns the amount you have accumulated after n years.
  This means you can live with (1-s)*y per year.
  
  If: 
    retirements(s,y,n,r) * (r-1) >= (1-s)*y
  Then:
    After n years, you can retire and withdraw (1-s)*y from your savings per 
    year and you'll never run out of cash. I.e. you can keep the same spending 
    style, without working. You can set that to any other threshold, but this 
    is the most interesting one for me.
  """
  return s * y * ((1+r)/2) * ((1-r**n)/(1-r))