import math

def divisors(n):
    lst = []
    for i in range(1, int(math.sqrt(n)) + 1):
        if n % i == 0:
            lst.append(i)
            if i != n // i:
                lst.append(n // i)
    lst.sort()
    return lst

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

for d in filter(lambda x: x != A and x != B, divisors(A + B)):
    if (A + d) % B == 0 and (B + d) % A == 0:
        print(d)
        break
else:
    print(-1)