For each pair $(A,B)$ of positive integers such that $A \times B \lt N$, a positive integer $C$ such that $A \times B + C = N$ is uniquely determined. Therefore, it is sufficient to count the number of such pairs of positive integers $(A,B)$.

When $A$ is fixed, the number of possible $B$ is $\left \lfloor \cfrac{N - 1}{A} \right \rfloor$. Therefore, we could solve the problem by performing exhaustive search on $A$ from $1$ to $N-1$.

Sample code in Python: https://atcoder.jp/contests/abc179/submissions/16844575