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<string>
#include<math.h>
#include<vector>
#include<algorithm>
#include<array>
#include<functional>
#include<iomanip>
#include<ios>
using namespace std;
#define FOR(i,n) for(int (i)=0;(i)<(n);(i)++)
const long long int mod=1e9+7;
typedef long long ll;
ll gcd(ll a, ll b) {
  if(a < b) return gcd(b, a);
  ll r;
  while ((r=a%b)) {
    a = b;
    b = r;
  }
  return b;
}
double reabs(double in){
    if(in<0){
        return (-1)*in;
    }else{
        return in;
    }
}

int main(){
    long long N,M;
    cin>>N>>M;

    long long K;

    if(N==M){
        K=0;
    }else{
        if((N+M)%2==0){
            K=abs(N+M)/2;
        }else{
            K=-1;
        }
    }
    if(K>=0){
        cout<<K<<endl;
    }else{
        cout<<"IMPOSSIBLE"<<endl;
    }

   
    return 0;
}