F(n) = F(i+j) = F(i-1)*F(j) + F(i)*F(j+1)

##</code><code>##

j = k**2 = int(sqrt(n))
i = n - j
F(n) = F(i+j) = F(i+k**2)

##</code><code>##

F(k**2) = (F(k-1)+F(k+1))*F(k*(k-1)) 
          - ((-1)**k)*F(k*(k-2))