Problem:
Score : 400 points

Problem Statement

Takahashi had a pair of two positive integers not exceeding N, (a,b), which he has forgotten.
He remembers that the remainder of a divided by b was greater than or equal to K.
Find the number of possible pairs that he may have had.

Constraints

1 \leq N \leq 10^5

0 \leq K \leq N-1

All input values are integers.

Input

Input is given from Standard Input in the following format:

N K

Output

Print the number of possible pairs that he may have had.

Sample Input 1

5 2

Sample Output 1

7

There are seven possible pairs: (2,3),(5,3),(2,4),(3,4),(2,5),(3,5) and (4,5).

Sample Input 2

10 0

Sample Output 2

100

Sample Input 3

31415 9265

Sample Output 3

287927211

Solution in Haskell (GHC 7.10.3) (judge verdict: Wrong Answer)

import Debug.Trace

main :: IO ()
main = do
    [n, k] <- (map read . words) <$> getLine
    print $ solv n k


solv :: Int -> Int -> Int
solv n k = noOfPairs n k

allPairs :: Int -> Int -> [(Int, Int)]
allPairs n k = do
    b <- [k+1..n]
    c <- [0..b-k-1]
    q <- [0..(n-k-c) `div` b]
    let a = b*q + k + c
    if a >= 1 then return (a,b) else []

noOfPairs :: Int -> Int -> Int
noOfPairs n k = sum $ do
    b <- [k+1..n]
    c <- [0..b-k-1]
    return $ (n-c-k) `div` b + 1

-- あるbに対して，あるcに対して，(100-c-k) `div` b + 1が(b, c)から求まる
-- (a,b)の組の数