Problem:
Score: 100 points

Problem Statement

We have two distinct integers A and B.

Print the integer K such that |A - K| = |B - K|.

If such an integer does not exist, print IMPOSSIBLE instead.

Constraints

All values in input are integers.

0 \leq A,\ B \leq 10^9

A and B are distinct.

Input

Input is given from Standard Input in the following format:

A B

Output

Print the integer K satisfying the condition.

If such an integer does not exist, print IMPOSSIBLE instead.

Sample Input 1

2 16

Sample Output 1

9

|2 - 9| = 7 and |16 - 9| = 7, so 9 satisfies the condition.

Sample Input 2

0 3

Sample Output 2

IMPOSSIBLE

No integer satisfies the condition.

Sample Input 3

998244353 99824435

Sample Output 3

549034394

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

import Data.Bits
import Data.List
import Control.Monad
import qualified Data.Vector.Unboxed as V
import qualified Data.Vector.Unboxed.Mutable as VM
import Control.Monad.ST
import Data.Maybe (fromMaybe)
import qualified Data.ByteString.Lazy.Char8 as B

main :: IO ()
main = do
  --s <- readLn :: IO Int
  [a',b'] <- map read.words <$> getLine :: IO [Int]
  let a = min a' b'
      b = max a' b'
  if even $ b - a then print $ a + ((b-a) `div` 2)
    else putStrLn $ "IMPOSSIBLE"