def gcd(a,b):
    if b==0:
        return a
    return gcd(b,a%b)
a,b=map(int,input().split())
d = gcd(a,b)
if d!=1:
    print(-1)
elif a==b==1:
    print(-1)
else:
    if b<a:
        a,b = b,a
    N = b*(a-1)
    A = [0]*N
    for k in range(a):
        for m in range((N-k*b)//a+1):
            if k*b+m*a>=N:break
            A[k*b+m*a] = 1
    ans = 0
    for i in range(1,N):
        if A[i]==0:
            ans += 1
    print(ans)