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 Ruby (2.3.3) (judge verdict: Accepted)

a, b = gets.split.map(&:to_i)
diff = (a - b).abs
ans = [a, b].min + (diff / 2)
j1 = a % 2 == 0 && b % 2 == 0
j2 = a % 2 != 0 && b % 2 != 0
if j1 || j2
    puts ans
else
    puts 'IMPOSSIBLE'
end