func readInts() -> [Int] {
    return readLine()!.split(separator: " ").map { Int($0)! }
}

func main() {
    let ab = readInts()
    let a = ab[0]
    let b = ab[1]
    
    if (a + b) % 2 == 0 {
        print((a + b) / 2)
    } else {
        print("IMPOSSIBLE")
    }
}

main()
