{-# OPTIONS_GHC -O2 #-}
{-# LANGUAGE BangPatterns          #-}
{-# LANGUAGE CPP                   #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE TupleSections         #-}
{-# LANGUAGE TypeFamilies          #-}

import           Control.Applicative
import           Control.Exception
import           Control.Monad
import           Control.Monad.Primitive
import           Control.Monad.ST
import           Control.Monad.State.Strict
import           Data.Bool
import qualified Data.ByteString.Char8       as B
import qualified Data.ByteString.Unsafe      as B
import           Data.Char
import qualified Data.Foldable               as F
import           Data.Function
import qualified Data.IntMap.Strict          as IM
import qualified Data.IntSet                 as IS
import           Data.List
import qualified Data.Map.Strict             as M
import           Data.Monoid
import           Data.Ord
import           Data.Primitive.MutVar
import qualified Data.Set                    as S
import           Data.Tuple
import qualified Data.Vector                 as V
import qualified Data.Vector.Generic         as G
import qualified Data.Vector.Generic.Mutable as GM
import qualified Data.Vector.Unboxed         as U
import qualified Data.Vector.Unboxed.Mutable as UM
import           Debug.Trace
import           Foreign                     hiding (void)
import           Unsafe.Coerce

main :: IO ()
main = do
    [n, m] <- map read.words <$> getLine :: IO [Int]
    xys <- U.unfoldrN n parseInt2 <$> B.getContents
    print $ solve m xys

solve :: Int -> U.Vector (Int, Int) -> Int
solve m xys = fst
    . U.foldl' step (0, m)
    . U.map decode
    . radixSort64
    $ U.map encode xys
  where
    step :: (Int, Int) -> (Int, Int) -> (Int, Int)
    step (!acc, !rest) (cost, num)
        | rest <= num = (acc + cost * rest, 0)
        | otherwise = (acc + cost * num, rest - num)

encode :: (Int, Int) -> Int
encode (x, y) = unsafeShiftL x 32 .|. y

decode :: Int -> (Int, Int)
decode x = (unsafeShiftR x 32, x .&. 0xffffffff)

-------------------------------------------------------------------------------
radixSort64 :: U.Vector Int -> U.Vector Int
radixSort64 v = foldl' step v [0, 16, 32, 48]
  where
    mask k x = fromIntegral $ unsafeShiftR x k .&. 0xffff
    step v k = U.create $ do
        pref <- U.unsafeThaw
            . U.prescanl' (+) 0
            . U.unsafeAccumulate (+) (U.replicate 0x10000 0)
            $ U.map ((,1) . mask k) v
        res <- UM.unsafeNew $ U.length v
        U.forM_ v $ \x -> do
            let !masked = mask k x
            i <- UM.unsafeRead pref masked
            UM.unsafeWrite pref masked $ i + 1
            UM.unsafeWrite res i x
        return res

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

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

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

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

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

Measured memory_bytes: 9468