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)

a, b = map(int, input().split())
def kansu_return_flag(a,b):
    ans = 0
    flag = 0

    if abs(a-b)%2 != 0:
        flag = 0
    else:
        ans = (a+b)/2
        flag = 1
    return flag,ans

flag,ans = kansu_return_flag(a,b)

if flag == 1:
    print(int(ans))
else:
    print('IMPOSSIBLE')