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 (3.8.2) (judge verdict: Runtime Error)

a,b = map(int,input().split())                                    │abc136  abc138  abc140  abc142  abc144  abc146  abc148
k = a+b                                                           │abc137  abc139  abc141  abc143  abc145  abc147  python
if k%2:                                                           │(base) rrif@rrif-dynabook-T65-EG:~/document/atcoder$ mkdir abc 13
    print('INPOSSIBLE')                                           │5
else:                                                             │(base) rrif@rrif-dynabook-T65-EG:~/document/atcoder$ rm -r abc
    print(k//2)