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)

s= list(map(int, input().split()))

if((s[0]-s[1])%2 == 1):
  print("IMPOSSIBLE")
  exit()

if(s[0]==s[1]):
  print(0)
  exit()

if(s[0]<s[1]):
  print(int(s[0]+(s[1]-s[0])/2))
else:
  print(int(s[1]+(s[0]-s[1])/2))