Problem:
Score : 400 points

Problem Statement

There are N one-off jobs available. If you take the i-th job and complete it, you will earn the reward of B_i after A_i days from the day you do it.

You can take and complete at most one of these jobs in a day.

However, you cannot retake a job that you have already done.

Find the maximum total reward that you can earn no later than M days from today.

You can already start working today.

Constraints

All values in input are integers.

1 \leq N \leq 10^5

1 \leq M \leq 10^5

1 \leq A_i \leq 10^5

1 \leq B_i \leq 10^4

Input

Input is given from Standard Input in the following format:

N M
A_1 B_1
A_2 B_2
\vdots
A_N B_N

Output

Print the maximum total reward that you can earn no later than M days from today.

Sample Input 1

3 4
4 3
4 1
2 2

Sample Output 1

5

You can earn the total reward of 5 by taking the jobs as follows:

Take and complete the first job today. You will earn the reward of 3 after four days from today.

Take and complete the third job tomorrow. You will earn the reward of 2 after two days from tomorrow, that is, after three days from today.

Sample Input 2

5 3
1 2
1 3
1 4
2 1
2 3

Sample Output 2

10

Sample Input 3

1 1
2 1

Sample Output 3

0

Solution in Haskell (GHC 7.10.3) (judge verdict: Time Limit Exceeded)

{-# OPTIONS_GHC -O2 #-}
{-# LANGUAGE OverloadedStrings #-}
{-# LANGUAGE BangPatterns #-}
module Main where

import Control.Arrow hiding ((+++))
import Control.Monad
import Control.Monad.ST
import Control.Monad.State.Strict
import Data.Char
import Data.Function
import Data.Graph
import Data.List
import qualified Data.Map.Strict as Map
import Data.Maybe
import qualified Data.Vector as V
import qualified Data.Vector.Mutable as VM
import qualified Data.Vector.Unboxed as U
import qualified Data.Vector.Unboxed.Mutable as UM
import qualified Data.ByteString.Char8 as C
import qualified Data.ByteString.Unsafe as B
import qualified Data.ByteString.Builder as BSB
import System.IO (stdout)

fst3 :: (a,b,c) -> a
fst3 (x,_,_) = x
snd3 :: (a,b,c) -> b
snd3 (_,y,_) = y
thd3 :: (a,b,c) -> c
thd3 (_,_,z) = z

type Parser a = C.ByteString -> Maybe (a, C.ByteString)

parseInt :: Parser Int
parseInt = C.readInt . C.dropWhile isSpace

parseInt2 :: Parser (Int, Int)
parseInt2 = runStateT $
    (,) <$> StateT (C.readInt . C.dropWhile isSpace)
        <*> StateT (C.readInt . B.unsafeTail)
 
parseInt3 :: Parser (Int, Int, Int)
parseInt3 = runStateT $
    (,,) <$> StateT (C.readInt . C.dropWhile isSpace)
        <*> StateT (C.readInt . B.unsafeTail)
        <*> StateT (C.readInt . B.unsafeTail)
 
parseInt4 :: Parser (Int, Int, Int, Int)
parseInt4 = runStateT $
    (,,,) <$> StateT (C.readInt . C.dropWhile isSpace)
        <*> StateT (C.readInt . B.unsafeTail)
        <*> StateT (C.readInt . B.unsafeTail)
        <*> StateT (C.readInt . B.unsafeTail)

-- prime means pred
parseInt' :: Parser Int
parseInt' = fmap (pred *** id) . parseInt

getInts :: IO [Int]
getInts = unfoldr parseInt <$> C.getLine

-- prime means pred
getInts' :: IO [Int]
getInts' = unfoldr parseInt' <$> C.getLine

getIntTuple :: IO (Int, Int)
getIntTuple = do
  a:b:_ <- getInts
  return (a, b)

-- prime means pred
getIntTuple' :: IO (Int, Int)
getIntTuple' = do
  a:b:_ <- getInts'
  return (a, b)

getIntTuple3 :: IO (Int, Int, Int)
getIntTuple3 = do
  a:b:c:_ <- getInts
  return (a, b, c)

getIntTuple4 :: IO (Int, Int, Int, Int)
getIntTuple4 = do
  a:b:c:d:_ <- getInts
  return (a, b, c, d)

getIntVec :: Int -> IO (U.Vector Int)
getIntVec n = U.unfoldrN n parseInt <$> C.getLine

-- prime means pred
getIntVec' :: Int -> IO (U.Vector Int)
getIntVec' n = U.unfoldrN n parseInt' <$> C.getLine

-- priority queue
-- https://stackoverflow.com/questions/6976559/comparison-of-priority-queue-implementations-in-haskell
--
data SkewHeap a = Empty
                | SkewNode a (SkewHeap a) (SkewHeap a)
                deriving (Show)

(+++) :: Ord a => SkewHeap a -> SkewHeap a -> SkewHeap a
heap1@(SkewNode x1 l1 r1) +++ heap2@(SkewNode x2 l2 r2) 
  | x1 >= x2   = SkewNode x1 (heap2 +++ r1) l1 
  | otherwise  = SkewNode x2 (heap1 +++ r2) l2
Empty +++ heap = heap
heap +++ Empty = heap

node :: a -> SkewHeap a
node x = SkewNode x Empty Empty

extractMax Empty = Nothing
extractMax (SkewNode x l r ) = Just (x , l +++ r )

fromList :: Ord a => [a] -> SkewHeap a
fromList = foldl' (+++) Empty . map node

foldSkewHeap :: b -> (a -> b -> b -> b) -> SkewHeap a -> b
foldSkewHeap c f = u
  where
    u Empty = c
    u (SkewNode a l r) = f a (u l) (u r)

sumSkewHeap :: SkewHeap Int -> Integer
sumSkewHeap = foldSkewHeap 0 (\a l r -> fromIntegral a + l + r)

---------------------------------------------------------

readProblem :: IO (Int, Int, [(Int, Int)])
readProblem = do
  (n, m) <- getIntTuple
  as <- replicateM n getIntTuple
  return (n, m, as)

solve :: (Integer, Map.Map Int (SkewHeap Int)) -> Int -> (Integer, Map.Map Int (SkewHeap Int))
solve prev@(!ttl, !map) !n = maybe prev (\(k, v, hp') -> (ttl+fromIntegral v, Map.alter (const (Just hp')) k map)) mv
  where
    !mv =  Map.foldlWithKey f Nothing map
    f !mv !k = maybe mv (\(v, hp') -> if k <= n && fmap snd3 mv <= Just v then Just (k, v, hp') else mv) . extractMax
  
main :: IO ()
main = do
  (n, m, as) <- readProblem
  let !map = foldl' (\m (k,v) -> Map.insertWithKey (\k nv ov -> nv +++ ov) k (node v) m) Map.empty as
  let (!ttl,_) = foldl' solve (0, map) [1..m]
  print ttl