Problem:
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.

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

Solution in PyPy3 (2.4.0) (judge verdict: Accepted)

#

import itertools
from collections import Counter
from collections import defaultdict
from collections import deque
import bisect
from heapq import heappush, heappop

def main():
    a, b = map(int, input().split())
    if abs(a - b) % 2 == 1:
        print('IMPOSSIBLE')
    else:
        dif = abs(a - b) // 2
        print(min(a,b) + dif)

if __name__ == '__main__':
    main()