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

def gcd(x, y):
  if y == 0: return x
  return gcd(y, x % y)

if gcd(A, B) != 1:
  print('-1')
else:
  can = [False] * (A * B + 1)
  for i in range(101):
    for j in range(101):
      if A*i + B*j <= A*B:
        can[A*i + B*j] = True
  print(can.count(False))