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<bits/stdc++.h> 
#define ll long long
#define pb push_back
#define mp make_pair
using namespace std;
ll gcd(ll a,ll b) 
{ 
	if (a == 0) 
		return b; 
	return gcd(b % a, a); 
} 
bool isprime(ll n)
{
    for(ll i=2;i<=sqrt(n);i++)
    {
        if(n%i==0)
            return false;
    }
    return true;
}
int main()
{
	ll a,b;
	cin>>a>>b;
	if((a+b)%2!=0)
	{
		cout<<"IMPOSSIBLE";
	}
	else
	{
		ll ans=(a+b)/2;
		cout<<ans;
	}
	return 0;
}