Problem:
Score : 100 points

Problem Statement

You went shopping to buy cakes and donuts with X yen (the currency of Japan).

First, you bought one cake for A yen at a cake shop.
Then, you bought as many donuts as possible for B yen each, at a donut shop.

How much do you have left after shopping?

Constraints

1 \leq A, B \leq 1 000

A + B \leq X \leq 10 000

X, A and B are integers.

Input

Input is given from Standard Input in the following format:

X
A
B

Output

Print the amount you have left after shopping.

Sample Input 1

1234
150
100

Sample Output 1

84

You have 1234 - 150 = 1084 yen left after buying a cake.
With this amount, you can buy 10 donuts, after which you have 84 yen left.

Sample Input 2

1000
108
108

Sample Output 2

28

Sample Input 3

579
123
456

Sample Output 3

0

Sample Input 4

7477
549
593

Sample Output 4

405

Solution in Haskell (GHC 7.10.3) (judge verdict: Accepted)

import Control.Applicative ((<$>), (<*>))
import Data.ByteString.Char8 (ByteString)
import qualified Data.ByteString.Char8 as B

main :: IO ()
main = do
  solve <$> getl (readi B.readInt) <*> getl (readi B.readInt) <*> getl (readi B.readInt) >>= print

solve :: Int -> Int -> Int -> Int
solve x a b = (x - a) `mod` b

fibs :: [Int]
fibs = 1:1:zipWith (+) (tail fibs) fibs

getl :: (ByteString -> a) -> IO a
getl f = f <$> B.getLine

readi :: Integral a =>  (ByteString -> Maybe (a, ByteString)) -> ByteString -> a
readi f s = let Just (n, _) = f s in n