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++14 (GCC 5.4.1) (judge verdict: Accepted)

#include <iostream>
#include <bits/stdc++.h>
#define IO ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0)
using namespace std;

int main()
{
    long long a,b;
    cin>>a>>b;
    if(a == b){
        cout<<0;
        return 0;
    }
    if(a > b)swap(a,b);
    if((b-a)%2 == 0){
        cout<<a+(b-a)/2;
    }else{
        cout<<"IMPOSSIBLE";
    }
    return 0;
}