import kotlin.math.sqrt

fun main() {
    val builder = StringBuilder()

    val (a, b) = readInputLine().split(" ").map { it.toInt() }

    var ans = -1

    for (c in 1..sqrt((a + b).toDouble()).toInt() + 1) {
        if (c == a || c == b) {
            continue
        }
        if ((a + b) % c == 0) {
            if ((a + c) % b == 0 && (b + c) % a == 0) {
                ans = c
                break
            }
        }
    }

    builder.appendln(ans)

    print(builder.toString())
}

fun readInputLine(): String {
    return readLine()!!
}