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-----
2 16

-----Sample Output-----
9

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

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