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: Accepted)

#!/usr/bin/env python3
# -*- coding: utf-8 -*-
# 問題：https://atcoder.jp/contests/abc135/tasks/abc135_a

a, b = map(int, input().strip().split())
diff = abs(a-b)
if diff % 2 == 0:
    print(int((a+b)/2))
else:
    print('IMPOSSIBLE')