import math

A, B = map(int, input().split())
ApB = A + B
divisor = [1, ApB]
index = 1
limit = int(math.sqrt(ApB))
for i in range(2, limit + 1):
    if ApB % i == 0 and i != A and i != B:
        divisor.append(i)
        N = ApB // i
        if N != i:
            divisor.append(N)
divisor.sort()

for C in divisor:
    if (A + C) % B == 0 and (B + C) % A == 0:
        print(C)
        break
else:
    print(-1)