n, k = [int(i) for i in input().strip().split(" ")]

ans = 0
if k == 0 or k > n:
    ans = 0
elif (n // 2 + 1) == k:
    if n == 2 and k == 2:
        ans = 0
    else:
        ans = n - 1
else:
    ans = n - 2

print(ans)