from math import gcd
a,b=map(int,input().split())
g=gcd(a,b)
if g!=1:
  print(-1)
else:
  S=set()
  for i in range(b+1):
    for j in range(a+1):
      if a*i+b*j<a*b:
        S.add(a*i+b*j)
  print(a*b-len(S))