Если увеличить интервал, то выдает ответ 1787     3200497 
  
  
 from functools import * 
 @lru_cache(None) 
  
 def F(n): 
   global a 
   a.append(n-5) 
   if n > 1: 
     a.append(n+8) 
     F(n-2) 
     F(n-3) 
   return a 
 a=[] 
  
 for n in range(1,1000): 
   F(n) 
   if sum(a)>3200000: 
     print(n,sum(a)) 
  
 

 def F(n):  
   global s  
   s += n - 5  
   if n > 1:  
     s += n + 8  
     F(n-2)  
     F(n-3)  
  
 for n in range(1,100):  
   s = 0 
   F(n)  
   if s > 3200000:  
     print(n, s)  
     break

  from functools import *  
  @lru_cache(None)  
  def F(n):  
    s = n-5  
    if n > 1:  
      s+= n+8 + F(n-2) + F(n-3)  
    return s