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

def f(A, B, C):
    res = True
    if (A+B)%C > 0:
        res = False
    if (B+C)%A > 0:
        res = False
    if (C+A)%B > 0:
        res = False
    return res

if B > A:
    A, B = B, A

if A != 2*B and f(A, B, A-B):
    ans = A-B
elif f(A, B, A+B):
    ans = A+B
else:
    ans = -1
print(ans)