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)

using namespace std;
#include <bits/stdc++.h>
#define rep(i, s) for (int i = 0; i < s; ++i)
#define repr(i, n) for(int i = n; i >= 0; i--)
#define FOR(i, m, n) for(int i = m; i < n; i++)
#define all(v) (v.begin(), v.end())
#define EACH(i, s) for (__typeof__((s).begin()) i = (s).begin(); i != (s).end(); ++i)
#define VEC(a, n) vector<int>a(n)
#define PQ(a) priority_queue<int>a
#define PQmin(a) priority_queue< int, :vector<int>, greater<int> >a
#define PAIR pair<int, int>
typedef long long ll;
typedef pair<ll, ll> l_l;
typedef pair<int, int> i_i;
#define EPS (1e-7)
#define INF (1e10)
#define PI (acos(-1))
const ll mod = 1000000007;
bool f(string s){
  bool ans=true;
  rep(i,s.size()/2){
    if(s[i]!=s[s.size()/2+i]){
      ans=false;
      break;
    }
  }
  return ans;
}

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

}