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 C (GCC 9.2.1) (judge verdict: Accepted)

int max(int a, int b)
{
  int n;
  if (a >= b)
    n = a;
  else
    n = b;
  return (n);
}

int main()
{
  int a, b;
  scanf("%d %d", &a, &b);
  if ((a - b) % 2 == 0)
    printf("%d", max(a, b) - abs((a - b) / 2));
  else
    printf("IMPOSSIBLE");
}