n, m = map(int,input().split())

res = None

if m >= n:
    res = 1
else:
    if n % 2 == 0:
        if m >= n // 2:
            reduce = m
            if reduce % 2 == 1:
                reduce -= 1
            rest = n // 2
            count = 1
            while rest > 0:
                rest -= reduce
                count += 1
            res = count
        else:
            res = -1
    else:
        res = -1
        
print(res)