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.chomp.split(" ").map(&:to_i)

if (a - b) % 2 == 1 then
  puts "IMPOSSIBLE"
  exit 0
end

k1 = (a - b) / 2
k2 = (b - a) / 2
k3 = (a + b) / 2

[k1, k2, k3].each do |k|
  if (a - k).abs == (b - k).abs then
    puts k
    exit 0
  end
end
puts "IMPOSSIBLE"