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 9.2.1) (judge verdict: Accepted)

#include <stdio.h>
#include <math.h>
#include<bits/stdc++.h>
using namespace std;
#define int long long int
 #define hi ios_base :: sync_with_stdio(false); cin.tie(0);cout.tie(0);
int i,j,k,mo;
main()

{
    int n,m;
    cin>>n>>m;
    if(n==m) cout<<0<<endl;
    else if((n+m)%2==0)
    {
        cout<<(n+m)/2<<endl;
    }
    else cout<<"IMPOSSIBLE"<<endl;
}