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)

main = do
  li <- getLine
  let [a,b] = map read $ words li
  let ans = compute a b
  case ans of
    Nothing -> putStrLn "IMPOSSIBLE"
    Just x -> print x

compute :: Int -> Int -> Maybe Int
compute a b
  | even d = Just m
  | True = Nothing
  where
    d = abs (a - b)
    m = (a + b) `div` 2