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 (Clang 3.8.0) (judge verdict: Runtime Error)

#include <stdio.h>

int main(){
  int n;
  long ans=0;
  scanf("%d",&n);
  
  int a[n+2],b[n];
  for(int i=1;i<=n+1;i++){
    scanf("%d",&a[i]);
  }
  for(int i=1;i<=n;i++){
    scanf("%d",&b[i]);
  }
  
  for(int i=1;i<=n;i++){
    if(a[i]<b[i]){
      if(a[i]+a[i+1]<=b[i]){
        ans=ans+a[i]+a[i+1];
        a[i+1]=0;
      }else{
        ans=ans+b[i];
        a[i+1]=a[i+1]-(b[i]-a[i]);
      }
    }else{
      ans=b[i];
    }
  }
  
  printf("%ld",ans);
  return 0;
}