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)

-- modules to import --
import qualified Data.ByteString.Char8
import qualified Data.Maybe
import           Prelude

-- functions for this task --
read_Integer = Prelude.fst . Data.Maybe.fromJust . Data.ByteString.Char8.readInteger
get_Integer  = read_Integer <$> Data.ByteString.Char8.getLine

-- main process is below --

task_A :: Integer -> Integer -> Integer -> Integer
task_A val_budget val_cake val_doughnut = mod (val_budget - val_cake) val_doughnut

main :: IO ()
main = do

        -- STEP.01
        -- read out the given data
        val_budget   <- get_Integer
        val_cake     <- get_Integer
        val_doughnut <- get_Integer

        -- STEP.02
        -- output the answer of this task
        Prelude.print $ task_A val_budget val_cake val_doughnut

-- End of this source code --