import java.util.Scanner

val sc = Scanner(System.`in`)

fun main(){
    var a = sc.nextInt()
    var b = sc.nextInt()
    
    if (a > b)
        a = b.also{ b = a }
    
    if (a*2 == b)
        println(a*3)
    else if (a*3 == b)
        println(a*2)
    else if (a*3 == b*2)
        println(a/2)
    else
        println(-1)
}