def divisor(num):
    div=[]
    for c in range(num):
        if num%(c+1)==0:
            div.append(c+1)
    return div
num1=input("plese input your two favorite number\n")
num2=input()
num=num1+num2
check=0
for c in divisor(num):
    if (c+num1)%num2==0 and (c+num2)%num1==0 and num1!=c and num2!=c:
        print(c)
        check+=1
        break
if check==0:
    print(-1)