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: Accepted)

module Main where
import qualified Control.Monad as C
import qualified Data.Char as C
import qualified Data.List as L
import qualified Data.Map as M
import qualified Data.Array as A

int = read :: String -> Int
getWord :: IO String
getWord = do
  c <- getChar
  if C.isSpace c
  then
    return []
  else do
    ws <- getWord
    return (c:ws)
getInt = fmap int getWord
replace pred a = map (\x -> if pred x then a else x)

main = interact $ printer . solver . parser
parser = int
printer = unlines . (:[]) . show

solver n = sum $ map (pattern n) [1..n]
    
pattern n a = (n-1) `div` a