N, K = list(map(int, input().split()))

box = list(range(N))

if K == 0 or K > N:
    print(0)
else:
    box[K-1] = None
    box[-K] = None
    if box[K-1] == box[-K] and box[K-1] == None:
        print(N - 1)
    else:
        print(N-2)