input_line = input().split()
a = int(input_line[0])
b = int(input_line[1])

max_n = 0
if(a > b):
    big_n = a
    small_n = b
else:
    big_n = b
    small_n = a


def check_c(c):
    if(((a+b) % c) == 0 and (b+c) % a == 0 and (c+a) % b == 0):
        return True
    else:
        return False


c = big_n - small_n
while(c <= big_n+small_n):
    if(c == big_n or c == small_n):
        continue
    elif(check_c(c)):
        print(c)
        exit()
    else:
        pass
    c += big_n

print(-1)