n, m = map(int, input().split())

if m >= n:
    print(1)
else:
    if n < 3:
        print(-1)
    else:
        min_k = (n + m - 1) // m  # Ceiling of n/m
        max_k = n // 3  # Floor of n/3
        if min_k <= max_k:
            print(min_k)
        else:
            print(-1)