from math import sqrt
 
def d(n):
  s = 1
  t = sqrt(n)
  for i in range(2, int(t)+1):
    if n % i == 0: s += i + n / i
  if t == int(t): s -= t
  return s
 
limit = 10000
sum = 0
for a in range(2, limit):
  d_a = d(a)
  if d_a > a and d(d_a) == a: sum += d_a + a 
 
print "Answer to PE21 = ",sum