def extend_gcd(m,n):
    x0,y0,x1,y1=1,0,0,1
    while n:
        q,m,n=m//n,n,m%n
        x0,x1=x1,x0-q*x1
        y0,y1=y1,y0-q*y1
    return x0,y0,m
#================================================
from math import gcd
A,B=map(int,input().split())

if gcd(A,B)>1:
    print(-1)
else:
    X=0
    s,t,_=extend_gcd(A,B)
    N=1
    while N*(A*s+B*t)<A*B:
        p=(-N*s+B-1)//B
        q=N*t//A
        if p>q:
            X+=1
        N+=1
    print(X)