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.