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)

-- A Harmony

main :: IO()
main = do
  l <- getLine
  let a: b: _ = take 2 (map read (words l) :: [Int])
  let ans = solve a b
  if ans < 0
    then
    putStrLn("IMPOSSIBLE")
    else
    putStrLn(show ans)

solve :: Int -> Int -> Int
solve a b =
  if even diff
  then
    diff_div + smaller_one
  else
    -1
  where
    smaller_one = min a b
    diff = abs (a - b)
    diff_div = diff `div` 2