N,K=map(int,input().split())
if N%2==0:
    if 0<K//2<=N//2:print(N-2)
    else:print(0)
else:
    if 0<K==(N//2+1):
        print(N-1)
    elif 0<K//2<=N//2 :
        print(N-2)
    else:
        print(0)