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())
i = 0
l = list()
while i * i <= A + B:
    i += 1
    if (A + B) % i != 0: continue
    l.append(i)
    l.append((A + B) // i)

l.sort()
for n in l:
    if n != A and n != B and (A + n) % B == 0 and (B + n) % A == 0:
        print(n)
        sys.exit()

print(-1)