import math
a,b=map(int,input().split())
c=math.gcd(a,b)
d=c
i=1
while c<=a+b:
    if (a+b)%c==0 and (a+c)%b==0 and (b+c)%a==0 and a!=b and b!=c and c!=a:
        print(c)
        break
    else:
        i+=1
        c=d*i
else:
    print(-1)