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 Go (1.6) (judge verdict: Accepted)

package main

import "fmt"

func main() {
  var a []int
  a = scanNums(2)
  if (a[0]+a[1])%2 == 0{
    fmt.Printf("%d\n",int((a[0]+a[1])/2))
  }else{
    fmt.Println("IMPOSSIBLE")
  }
}

func scanNums(len int) (nums []int) {
    var num int
    for i := 0; i < len; i++ {
        fmt.Scan(&num)
        nums = append(nums, num)
    }
    return
}