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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.

2 16

### Sample Output 1

9

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

0 3

### Sample Output 2

IMPOSSIBLE

No integer satisfies the condition.

### Sample Input 3

998244353 99824435

549034394