CN-SMO 2020 Problem 3

Given a polynomial $f(x)=x^{2020}+\sum_{i=0}^{2019} c_ix^i$ , where $c_i \in \{ -1,0,1 \}$ . Denote $N$ the number of positive integer roots of $f(x)=0$ (counting multiplicity). If $f(x)=0$ has no negative integer roots, find the maximum of $N$ .