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 (Clang 3.8.0) (judge verdict: Accepted)

#include <iostream>
#include <vector>
#include <queue>
#include <deque>
#include <math.h>
#include <stdlib.h>
#include <algorithm>
#define rep(i,n) for (int i = 0; i < (n); ++i)
using namespace std;
typedef long long ll;

template<class T> inline bool chmin(T &a, T b) { if(a > b){ a = b; return true; } return false; }
template<class T> inline bool chmax(T &a, T b) { if(a < b){ a = b; return true; } return false; }

//INT_MAX	2,147,483,647 = 2*1.0e9

int main(){
  int A, B;
  cin >> A >> B;
  int sum = A+B;
  if(sum%2 == 1){
    cout << "IMPOSSIBLE" << endl;
  } else {
    cout << sum/2 << endl;
  }

  return 0;
}