import math
a, b = map(int, input().split())
if math.gcd(a, b) != 1:
    print(-1)
else:
    MAX = int(1e4)
    dp = [1] * (MAX + 1)
    for i in range(MAX // a + 1):
        for j in range(MAX // b + 1):
            if a * i + b * j <= MAX:
                dp[a * i + b * j] = 0
    print(sum(dp))