import sys
read = sys.stdin.buffer.read
readline = sys.stdin.buffer.readline
readlines = sys.stdin.buffer.readlines

"""
・全員の団体行動
・ちょうど半分
"""

N,M = map(int,read().split())

if N <= M:
    answer = 1
elif N%2 == 0 and M >= N//2:
    answer = 2
else:
    answer = -1
print(answer)