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

def gcd(a, b):
    if a < b:
        a, b = b, a
    if a%b == 0:
        return b
    return gcd(b, a%b)

if gcd(A, B) > 1:
    ans = -1
else:
    M = 10**5
    memo = [0]*M
    ans = 0
    cnt = 0
    for i in range(1, M):
        if i%A == 0 or i%B == 0:
            memo[i] = 1
            cnt += 1
        elif i-A > 0 and memo[i-A] == 1:
            memo[i] = 1
            cnt += 1
        elif i-B > 0 and memo[i-B] == 1:
            memo[i] = 1
            cnt += 1
        else:
            ans += 1
            cnt = 0
        if cnt == min(A, B):
            break

print(ans)