n, k = map(int, input().split())
if k == 0:
    print(0)
    exit()
if k > n:
    print(0)
    exit()
if n%2 == 1:
    if k == (n+1)//2:
        print(n-1)
    else:
        print(n-2)
else:
    print(n-2)