n=raw_input()
N=map(int, n.split())

if N[0]<N[1] or N[1]==0 or N[1]==N[0]==2 or N[1]==N[0]==1:
    print 0
elif N[1]==(N[0]/2)+1 and N[0]%2==1:
    print N[0]-1

else:
    print N[0]-2