#include <stdio.h>
#include <algorithm>
#include <iostream>
#include <string>
#include <vector>
#include <functional>
#include <map>
#include <iomanip>
#include <math.h> 
#include <stack>
#include <queue>
#include <bitset>
#include <cstdlib>
#include <tuple>
#include <cctype>
#include <ctype.h>
#include <set>
#include <sstream>

using namespace std;



int main() {
	int i, j;
	long long n, k;


	cin >> n >> k;

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









	getchar();
	getchar();
	return 0;
}