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)

def main():
    i = list(map(int, input().split()))

    for j in i:
        if isinstance(j, int) == False:
            return
        if j < 0 or 10**9 < j:
            return
    if i[0] == i[1]:
        return

    a = i[0]
    b = i[1]

    k = (a + b)/2

    if k*2 % 2 == 0:
        print(int(k))
    else:
        print('IMPOSSIBLE')

if __name__ == '__main__':
    main()