import sys
readline = sys.stdin.readline

# どちらかが1であれば0
# 互いに素でなければ-1
# 互いに素なら、公倍数までの作れない数を数える

A,B = map(int,readline().split())
if A == 1 or B == 1:
  print(0)
  exit(0)
  
import math
g = math.gcd(A,B)
if g != 1:
  print(-1)
  exit(0)
  
L = A // g * B

ans = 0
for i in range(1, L + 1):
  for a in range(L // A + 1):
    candi = i - a * A
    if (candi == 0) or (candi > 0 and candi % B == 0):
      break
  else:
    ans += 1

print(ans)