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




if k==0:
    print(0)
    exit()
elif k>n:
    print(0)
    exit()
    
else:
    if n%2==0:
        print(n-2)
        exit()
    elif n%2==1:
        if k==1+n//2:
            print(n-1)
            exit()
        else:
            print(n-2)
            exit()