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