#define _USE_MATH_DEFINES
#define _CRT_SECURE_NO_WARNINGS
#include<stdio.h>
#include<string>
#include<iostream>
#include<iostream>
#include<cctype>
#include<cstdio>
#include<vector>
#include<stack>
#include<queue>
#include <algorithm>
#include<math.h>
#include<set>
#include<map>
#include <sstream>
#include<iomanip>
#include <ctype.h>
using namespace std;

//#include<bits/stdc++.h>



int main() {
	
	long long int n, k;
	cin >> n >> k;
	if (!k || k > n)cout << 0 << endl;
	else if(n%2&&k==n/2+1) cout << n - 1 << endl;
	else cout << n - 2 << endl;
	
	

	return 0;
}