import sys

def main():
    n, m = map(int, input().split())

    if n <= m:
        print(1)
        sys.exit()

    if n % 2 != 0:
        print(-1)
        sys.exit()

    if n % 2 == 0:
        if m >= n // 2:
            print(2)
        else:
            print(-1)

if __name__ == '__main__':
    main()