n,m = map(int,input().split(" "))
days_for_move = 0

if n < m :
    days_for_move = 1

if n %2 == 0:
    if n/2 <= m :
        days_for_move = 2
    else:
        days_for_move = -1
else :
    days_for_move = -1

print(days_for_move)