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<iomanip>

#include<vector>
#include<queue>
#include<set>
#include<map>
#include<string>

#include<algorithm>
#include<cmath>

using namespace std;
typedef long long ll;
template <class T> inline bool chmax(T& a, T b) {if (a < b) {a = b; return 1;} return 0;}
template <class T> inline bool chmin(T& a, T b) {if (a > b) {a = b; return 1;} return 0;}
const long long INF = 1LL << 60;
typedef pair<int, int> P;

int main() {
    int a, b;
    cin >> a >> b;
    int tmp = abs(a - b);
    if (tmp % 2 == 1) {
        cout << "IMPOSSIBLE\n";
    } else {
        cout << (a+b)/2 << "\n";
    }
}