N,k=map(int,input().split())
if k%2==0:
    ifPress_1=set(range(1,2+k,2)) # start from One
    ifPress_2=set(range(N-k+1,N,2))
else:
    ifPress_1=set(range(2,2+k,2)) # start from One
    ifPress_2=set(range(N-k+1,N+1,2))
print(len(ifPress_1|ifPress_2))