from math import gcd
a,b = map(int,input().split())
if gcd(a,b) > 1:
    print(-1)
else:
    count = [1]*(10**6)
    for i in range(500):
        for j in range(500):
            count[i*a+b*j] = 0
            
    print(sum(count[:a*b//gcd(a,b)]))