from sys import stdin
import sys
import math
##  input functions for me
def ria(sep = ''):
    if sep == '' :
        return list(map(int, input().split())) 
    else: return list(map(int, input().split(sep)))
def rsa(sep = ''):
    if sep == '' :
        return input().split() 
    else: return input().split(sep)
def ri(): return int(input())
def rd(): return float(input())
def rs(): return input()
##

## main ##

A, B = map(int, input().split())
ans = -1
for i in range(1,int(math.sqrt(A + B) + 1)):
    if((A + B) % i != 0): continue
    if( (i + B) % A == 0 and (i + A) % B == 0 and i != A and i != B):
        ans = i
        break
if ans == -1 and (A + B + B) % A == 0 and (A + A + B) % B == 0 : ans = A + B
print(ans)