# -*- coding: utf-8 -*-
"""
No.521 Cheeses and a Mousetrap(チーズとネズミ捕り)
https://yukicoder.me/problems/no/521
"""
import sys
from sys import stdin
input = stdin.readline


def solve(N, K):
    if K == 0 or K > N:
        return 0
    if K*2 == N+1:
        return N - 1
    return N - 2


def main(args):
    N, K = map(int, input().split())
    ans = solve(N, K)
    print(ans)


if __name__ == '__main__':
    main(sys.argv[1:])