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: Wrong Answer)

# -*- coding: utf-8 -*-

inputAB = input()
inputAB.split()

inputA = int(inputAB[0])
inputB = int(inputAB[2])

if inputA > inputB:
    largeNum = inputA
    smallNum = inputB
else:
    largeNum = inputB
    smallNum = inputA


if ((largeNum - smallNum) % 2) == 1:
    print("IMPOSSIBLE")
else:
    answer = int(smallNum + (largeNum - smallNum) / 2)
    print(answer)