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)

def main()

    # stdin = $stdin.readlines.map {|i| i.chomp().to_i }
    stdin =  gets.chomp.split(" ").map {|i| i.chomp().to_i }
    diff = (stdin[0]-stdin[1]).abs 
    if diff%2 == 0
    print (stdin[0]-diff/2).abs
    else
        print "IMPOSSIBLE"
    end
    


end
main()