from math import gcd
a,b = map(int,input().split())
if a>b:
    a,b = b,a
if a==0:
    print(0)
elif gcd(a,b)!=1:
    print(-1)
else:
    k = a*b//gcd(a,b)
    f =[0]*(k+1)
    for i in range(k//a+1):
        for j in range(k//b+1):
            l = a*i + b*j
            if l<=k:
                f[l] = 1
    print(f.count(0))