#include<iostream>
#include<string>
#include<string.h>
#include<algorithm>
#include<stdio.h>
#include<cmath>
#include<vector>
#include<utility>
#include<stack>
#include<queue>
#include<list>
#include<bitset>
#include<functional>

#define FOR(i, a, b) for(int i=(a);i<=(b);i++)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define MOD 1000000007
#define INF 1000000000
#define PI 3.14159265358979

using namespace std;
typedef pair<int, int> P;

int main(void) {
	long long int n, k;

	cin >> n >> k;

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