import strutils

proc input: string =
  while true:
    let t = stdin.readChar
    if t.isSpaceAscii: break else: result.safeAdd t

let N, K = input().parseInt
var ans = 0

if K != 0 and K <= N:
  ans += N - 2
  if N mod 2 == 1 and (N + 1) div 2 == K:
    ans += 1

echo ans