from math import ceil


def main():
    n, m = map(int, input().split())
    if n <= m:
        print(1)
        return

    if m == 1:
        if n == 2:
            print(2)
        else:
            print(-1)
        return
    
    if n == 3:
        print(-1)
        return

    if n % 2 == 0:
        print(-1)
        return

    if ceil(n / 2) <= m:
        print(2)
    else:
        print(-1)


if __name__ == "__main__":
    main()