#include<iostream>
#include<string>
#include<algorithm>
#include<functional>
#include<vector>
#include<set>
#include<queue>
#include<list>
#include<stdio.h>
#include<stdlib.h>
#include<map>

#define rep(i,a,b) for(int i=a;i<b;i++)
#define rrep(i,a,b) for(int i=a;i>=b;i--)
#define fore(i,a) for(auto &i:a)
#define all(x) (x).begin(),(x).end()

typedef long long ll;

using namespace std;

int main() {
	
	int n, k;
	cin >> n >> k;

	if (k==0 || k>n) {
		cout << "0" << endl;
		return 0;
	}

	if (n % 2 == 0) {
		cout << n - 2 << endl;
		return 0;
	}

	if (n % 2 == 1 && k == (n+1) / 2) {
		cout << n - 1 << endl;
	}
	else {
		cout << n - 2 << endl;
	}
		
	return 0;
}