input: Write a python function to find the sum of divisors of a given number.
  assert sum_of_divisors(24) == 60
  assert sum_of_divisors(12) == 28
  assert sum_of_divisors(1) == 1
  assert sum_of_divisors(7) == 8
output: def sum_of_divisors(n):
      total = 0
      for i in range(1, n + 1):
          if n % i == 0:
              total += i
      return total