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(){
  //入力
  int A,B,K;
  //int型で約2.1*10^9が最大値からok、又KはAとBの平均値からこれもok
  scanf("%d %d", &A,&B);

  //計算
  K = (A+B)/2;
  //A,Bは相異なる整数から、A-KとB-Kの符号が一致する事はない。
  //よってA<Bとすれば、A<K<Bであり、K=A+B/2となる。
  //よってKが整数の時にこれを出力すれば良い。

  //出力
  if((A+B)%2 == 0){
    printf("%d", K);
  }else{
    printf("IMPOSSIBLE");
  }
  return 0;
}