#!/usr/bin/env python3
# from typing import *

import sys
import io
import math
import collections
import decimal
import itertools
import bisect
import heapq


def input():
    return sys.stdin.readline()[:-1]


# sys.setrecursionlimit(1000000)

# _INPUT = """# paste here...
# """
# sys.stdin = io.StringIO(_INPUT)

INF = 10**10

def get_divisors(n):
    lower_divisors = []
    upper_divisors = []
    i = 1
    while i * i <= n:
        if n % i == 0:
            lower_divisors.append(i)
            if i != n // i:
                upper_divisors.append(n//i)
        i += 1
    return lower_divisors + upper_divisors[::-1]

# def solve(A: int, B: int) -> int:
def solve(A, B):
    s = A+B
    divs = get_divisors(s)
    for d in divs:
        if d != A and d != B and (A+d) % B == 0 and (B+d) % A == 0:
            return d
    return -1



A, B = map(int, input().split())
print(solve(A, B))