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>
using namespace std;
typedef long long  ll;
typedef vector<int> VI;
typedef vector<ll> VLL;

int main(){
    ll a,b;
    cin >> a >> b;

    if(abs(a-b)% 2 != 0 ){
        cout << "IMPOSSIBLE" << endl;
        return 0; 
    }else{
        ll mi = min(a,b);
        ll tmp = abs(a-b)/2;
        cout << mi + tmp << endl;
        return 0;
    }

}