import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

from fractions import gcd

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

g = gcd(A,B)
a = A//g
b = B//g

c = 6 - a - b

if sorted((a,b,c)) == [1,2,3]:
    print(c * g)
else:
    print(-1)