import  sys

A,B = map(int,input().split())

def gcd(a,b):
    while True:
        r = a % b
        a = b
        b = r
        if r == 0:
            return a
d = gcd(A,B)
if d != 1:
    print(-1)
    exit()
dat = [1] * 20000
for x in range(20000):
    for y in range(20000):
        c = A * x + B * y
        if c < 20000:
            dat[c] = 0
print(sum(dat))