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 Java (OpenJDK 11.0.6) (judge verdict: Accepted)

import java.io.*;
// import java.math.*;
// import java.util.*;
// import java.util.Map.*;
// import java.util.stream.*;

public class Main {
    public static void main (String[] args) throws Exception {
        BufferedReader reader =
            new BufferedReader (new InputStreamReader (System.in));
        String[] arr = reader.readLine().split (" ");
        int A, B;
        A = Integer.parseInt (arr[0]);
        B = Integer.parseInt (arr[1]);
        System.out.println ((A + B) % 2 == 0 ? (A + B) / 2 : "IMPOSSIBLE");
        return;
    }
}