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: Wrong Answer)

#include <bits/stdc++.h>
using namespace std ;
int main()
{
    long long a , b , c , d , x=0 , i ;
    bool w = true ;
    cin  >> a >> b ;
    for(i=1 ; i <= 1000000 ; i++){
        c = fabs(a-i) ;
        d = fabs(b-i) ;
        if(c == d){
            cout << i ;
        return 0 ;
        }else
            continue;

    }
        cout << "IMPOSSIBLE" ;

    return 0 ;
}