Problem:
Score : 300 points

Problem Statement

Given is a positive integer N.
How many tuples (A,B,C) of positive integers satisfy A \times B + C = N?

Constraints

2 \leq N \leq 10^6

All values in input are integers.

Input

Input is given from Standard Input in the following format:

N

Output

Print the answer.

Sample Input 1

3

Sample Output 1

3

There are 3 tuples of integers that satisfy A \times B + C = 3: (A, B, C) = (1, 1, 2), (1, 2, 1), (2, 1, 1).

Sample Input 2

100

Sample Output 2

473

Sample Input 3

1000000

Sample Output 3

13969985

Solution in Haskell (GHC 8.8.3) (judge verdict: Time Limit Exceeded)

import           Data.List

inputN :: IO Int
inputN = readLn

divisor :: Int -> [Int]
divisor n = filter (\x -> mod n x == 0) [1..n]

multNum :: [Int] -> Int
multNum ns = length ns

func :: Int -> Int
func n = multNum . divisor $ n

go :: Int -> Int
go n = foldl' (+) 0 $ map func $ [1..(n-1)]

main :: IO ()
main = do
  n <- inputN
  putStrLn . show . go $ n
  return ()