### BUGGY CODE ###

import math
import queue
import bisect
import heapq
import time
import itertools

mod = int(1e9+7)

def swap(a,b):
    return (b,a)

def my_round(a,dig=0):
    p = 10 ** dig
    return (a * p * 2 + 1) // 2 / p

def gcd(a,b): #最大公約数
    if (a<b):
        a,b = swap(a,b)
    if (b==0):
        return a
    else:
        return gcd(b,a%b)

def lcm(a,b): #最小公倍数
    return a/gcd(a,b)*b

def divisors(a): # 約数列挙
    divisors = []
    for i in range(1,int(a**0.5)+1):
        if a%i==0:
            divisors.append(i)
            if i!=a//i:
                divisors.append(a//i)
    return divisors

def is_prime(a): #素数判定
    if a<2:
        return False
    elif a==2:
        return True
    elif a%2==0:
        return False
    sqrt_num = int(a**0.5)
    for i in range(3,sqrt_num+1,2):
        if a%i==0:
            return False
    return True

def prime_num(a): #素数列挙
    pn = [2]
    for i in range(3,int(a**0.5),2):
        prime = True
        for j in pn:
            if i%j==0:
                prime = False
                break
        if prime:
            pn.append(i)
    return pn

def prime_fact(a): #素因数分解
    sqrt = math.sqrt(a)
    res = []
    i = 2
    if is_prime(a):
        res.append(a)
    else:
        while a!=1:
            while a%i==0:
                res.append(i)
                a //= i
            i += 1
    return res

def main():
    x,y = map(int,input().split())
    num = x * 2
    while x < int(1e18):
        if num % y != 0:
            print(num)
            return
        num += x
    print(-1)
    return

if __name__=='__main__':
    main()


### FIXED CODE ###

import math
import queue
import bisect
import heapq
import time
import itertools

mod = int(1e9+7)

def swap(a,b):
    return (b,a)

def my_round(a,dig=0):
    p = 10 ** dig
    return (a * p * 2 + 1) // 2 / p

def gcd(a,b): #最大公約数
    if (a<b):
        a,b = swap(a,b)
    if (b==0):
        return a
    else:
        return gcd(b,a%b)

def lcm(a,b): #最小公倍数
    return a/gcd(a,b)*b

def divisors(a): # 約数列挙
    divisors = []
    for i in range(1,int(a**0.5)+1):
        if a%i==0:
            divisors.append(i)
            if i!=a//i:
                divisors.append(a//i)
    return divisors

def is_prime(a): #素数判定
    if a<2:
        return False
    elif a==2:
        return True
    elif a%2==0:
        return False
    sqrt_num = int(a**0.5)
    for i in range(3,sqrt_num+1,2):
        if a%i==0:
            return False
    return True

def prime_num(a): #素数列挙
    pn = [2]
    for i in range(3,int(a**0.5),2):
        prime = True
        for j in pn:
            if i%j==0:
                prime = False
                break
        if prime:
            pn.append(i)
    return pn

def prime_fact(a): #素因数分解
    sqrt = math.sqrt(a)
    res = []
    i = 2
    if is_prime(a):
        res.append(a)
    else:
        while a!=1:
            while a%i==0:
                res.append(i)
                a //= i
            i += 1
    return res

def main():
    x,y = map(int,input().split())
    if x == y:
        print(-1)
        return
    if y % x == 0:
        mul = y // x + 1
        print(x * mul)
        return
    for i in range(1,4):
        if (x * i) % y != 0:
            print(x * i)
            return
    print(-1)
    return

if __name__=='__main__':
    main()