n,k=map(int,input().split())
import math
if k==0 or n<k:
    print(0)
else:
    if n%2!=0 and math.ceil(n/2)==k:
        print(n-1)
    elif n%2!=0 and math.ceil(n/2)!=k:
        print(n-2)
    else:
        print(n-2)