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)

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import sys
from fractions import gcd

input = sys.stdin.readline

def main():
    A, B = map(int, input().split())
    max_n = max(A, B)
    min_n = min(A, B)
    if (max_n - min_n) % 2 == 1:
        print("IMPOSSIBLE")
        return
    else:
        print(int(min_n + (max_n - min_n) / 2))

main()