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 int LL;
typedef long long int LLint;
typedef pair<int,int> intpair;
typedef pair<bool,bool> boolpair;
typedef pair<string,string> strpair;
typedef pair<LL,LL> LLpair;
typedef pair<double,double> doublepair;
typedef pair<float,float> floatpair;
typedef vector<int> intvector;
typedef vector<bool> boolvector;
typedef vector<string> strvector;
typedef vector<LL> LLvector;
typedef vector<double> doublevector;
typedef vector<float> floatvector;
#define wait(sec) usleep((sec) * 1000000)
#define ED return 0;
#define TEST cout << "OK" << endl;
#define UP(a,b) ((a+(b-1))/b)
#define SORT(vec) sort(vec.begin(),vec.end());
#define DOUBLECHANGE(count) cout << setprecision(count);
#define REV(vec) reverse(vec.begin(),vec.end());
#define ipow(x,y) LL(pow(x,y))
#define INF 999999999
const long long mod = 1000000007;

int main(){
//    random_device rnd;
//    mt19937 mt(rnd());
      cin.tie(0);
      ios::sync_with_stdio(false);
      int A, B;
      cin >> A >> B;
      if((A+B) % 2 == 0){
          cout << (A+B)/2 << endl;
      }
      else{
          cout << "IMPOSSIBLE" << endl;
      }
}