#include <cmath>
#include <string>
#include <vector>
#include <iomanip>
#include <iostream>
#include <algorithm>
#include <functional>
#pragma warning(disable : 4996)
using namespace std;
int n, m;
int main() {
	cin >> n >> m;
	if (n + 1 == m * 2) cout << n - 1 << endl;
	else if (1 <= m && m <= n) cout << n - 2 << endl;
	else cout << 0 << endl;
	return 0;
}