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 Python (3.4.3) (judge verdict: Accepted)

import sys
A, B = map(int, input().split())
tmp = 0

if (abs(A)-abs(B))%2 == 1:
  print("IMPOSSIBLE")
  sys.exit()

if A < B:
  tmp = A
  A = B
  B = tmp

if A > 0 and B > 0:
  K = (A+B)/2
elif A < 0 and B < 0:
  K = -(abs(A)+abs(B))/2
else:
  K = A-(abs(A)+abs(B))/2

print(int(K))