# coding: utf-8 import array, bisect, collections, copy, heapq, itertools, math, random, re, string, sys, time sys.setrecursionlimit(10 ** 7) INF = 10 ** 20 MOD = 10 ** 9 + 7 def II(): return int(input()) def ILI(): return list(map(int, input().split())) def IAI(LINE): return [ILI() for __ in range(LINE)] def IDI(): return {key: value for key, value in ILI()} def read(): N, K = ILI() return (N, K) def solve(N, K): ans = 0 if K <= 0 or K >= N: ans = 0 else: if N % 2 == 1 and K == N // 2 + 1: ans = N - 1 else: ans = N - 2 return ans def main(): params = read() print(solve(*params)) if __name__ == "__main__": main()