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 5.4.1) (judge verdict: Accepted)

#include <stdio.h>

int main() {
  int a,b,c,d,k;
  c = 0;
  d = 0;
  scanf("%d %d", &a, &b);
  for(int i = 0; i < 1000000000; i++) {
    c = a - i;
    d = b - i;
    if (c<0){
      c = -1 * c;
    }
    if (d<0) {
      d = -1 * d;
    }
    if (c == d) {
      k = i;
      printf("%d", k);
      return 0;
      break;
    }
  }
  printf("IMPOSSIBLE");
  return 0;
}