package main

import "fmt"

func main() {
	var a, b int
	fmt.Scan(&a, &b)
	if (a+b)%2 == 1 {
		fmt.Println("IMPOSSIBLE")
		return
	}
	if b > a {
		fmt.Println(b - (b-a)/2)
		return
	}
	fmt.Println(a - (a-b)/2)
}
