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: Runtime Error)

n = int(input())
a = list(map(int,input().split()))
b = list(map(int,input().split()))
c = 0
for i in range(n):
  if a[i]>=b[i]:
    c+=int(b[i])
  else:
    if (a[i]+a[i+1])>=b[i]:
      c+=int(b[i])
      a[i+1]=a[i]+a[i+1]-b[i]
    else:
      c+=int(a[i]+a[i+1])
print(c)