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 Python (3.4.3) (judge verdict: Accepted)

import sys

A, B = map(int, input().split())
K = [0]*4
K[0] = (A+B)/2
K[1] = (A-B)/2
K[2] = -(A+B)/2
K[3] = -(A-B)/2

for i in K:
    if abs(A-i) == abs(B-i):
        if int(i) == i:
            print(int(i))
            sys.exit()

print('IMPOSSIBLE')