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: Runtime Error)

n = gets.to_i
m = gets.split(" ").map(&:to_i)
h = gets.split(" ").map(&:to_i)

t = 0

n.times do |i|
  if m[i] <= h[i]
    t += m[i]
    h[i] -= m[i]
    if m[i + 1] <= h[i]
      t += m[i + 1]
      m[i + 1] = 0
    else
      t += h[i]
      m[i + 1] -= h[i]
      h[i] = 0
    end
  else
    t += h[i]
  end

 puts t