# -*- coding: utf-8 -*-
"""
No.531 エヌスクミ島の平和協定
https://yukicoder.me/problems/no/531

"""
import sys
from sys import stdin
input = stdin.readline


def main(args):
    n, m = map(int, input().split())
    if n <= m:
        print(1)
    elif n % 2 == 1:
        print(-1)
    elif n <= m*2:
        print(2)
    else:
        print(-1)


if __name__ == '__main__':
    main(sys.argv[1:])