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.

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.

Constraints

• All values in input are integers.
• $0 \leq A$,\ $B \leq 10^9$
• A and B are distinct.

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

见题面 Input 部分

见题面 Output 部分