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 - k| = |b - k|
# a2 - 2ak + k2 = b2 - 2bk + k2
# a2 - 2ak = b2 - 2bk
# 2ak - 2bk = a2 - b2
# 2k(a - b) = a2 - b2
# k = (a2 - b2) / 2(a - b)
# k = (a + b) / 2

a, b = readline.strip.split.map(&:to_i)
if (a + b) % 2 == 0
  puts (a + b) / 2
else
  puts 'IMPOSSIBLE'
end