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: Wrong Answer)

s = gets.chomp!.split("").map{|x| x == '?' ? -1 : x.to_i}
l = s.size
dp = Array.new(l+1).map{Array.new(13,0)}
dp[0][0] = 1

l.times do |i|
    c = s[i]
    10.times do |key|
        if c == -1 || c == key then
            13.times do |j|
                dp[i+1][(j*10+key) % 13] += dp[i][j] % (10 ** 9 + 7) 
            end
        end
    end
end

puts dp[l][5] % (10 ** 9 + 7)