#yuki816
import sys

a,b=map(int,raw_input().split())
if 2*a==b:
 c=3*a
elif 3*a==b:
 c=2*a
elif 2*b==a:
 c=3*b
elif 3*b==a:
 c=2*b
elif 3*a==2*b:
 c=a/2
elif 3*b==2*a:
 c=b/2
else:
 print -1
 sys.exit()
print c