def divisor(num):
    div=[]
    for c in range(num):
        if num%(c+1)==0:
            div.append(c+1)
    return div
s=list(map(int,input().split()))
num1=s[0]
num2=s[1]
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)