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: Wrong Answer)

#include <stdio.h>
int main(){
  int K, A, B;
  int X, Y, N;
  scanf("%d %d", &A, &B);
  if(A<B){
    N = B - A;
  }else{
    N = A - B;
  }
  for(K=0; K<N-1; K++){
    X = A - K;
    Y = B - K;
    if(X < 0) X = -X;
    if(Y < 0) Y = -Y;
    if(X == Y){
      break;
    }else{
      X=0;
      Y=0;
    }
  }
  if ( X == 0 && Y == 0) printf("IMPOSSIBLE");
  else if(X == Y) printf("%d", K);
  return 0;
}