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 - Harmony
#
# 2分探索法

a, b = gets.split.map(&:to_i)


if a<b
  tmp = a
  a = b
  b = tmp
end



def binary_search(a, b)

  head = b
  tail = a

  while head <= tail

    center = (head + tail) / 2

    if (a-center).abs == (b-center).abs
      return "#{center}"
    elsif (b-center).abs < (a-center).abs
      head = center + 1
    else
      tail = center - 1
    end

  end

  return "IMPOSSIBLE"

end

puts binary_search(a, b)