#ifdef __GNUC__ 
#pragma GCC optimize(2)
#endif

#include <cassert>
#include <iostream>
#include <functional>
#include <iomanip>
#include <set>
#include <numeric>
#include <unordered_map>
#include <map>
#include <queue>
#include <stack>
#include <complex>
#include <vector>
#include <deque>
#include <algorithm>
#include  <cstring>
#include <string>
#include <cmath> 
#include <random>

#ifdef _MSC_VER
#pragma warning(disable: 4996)
#pragma warning (error: 4715 4700 4703 4533 4789 4717) 
#endif

using namespace std;

namespace Bigcat {
	template <typename T> inline void max_s(T&a, const T&b) { a = max(a, b); }
	template <typename T> inline void min_s(T&a, const T&b) { a = min(a, b); }

	template <typename T>  void sort_v(T & x) { sort(x.begin(), x.end()); }
	template <typename T>  void erase_dup(T & x) { x.erase(unique(x.begin(), x.end()), x.end()); }    // erase duplicate
	template <typename T, typename V>  int pos_l(const T & x, const V & v) { return lower_bound(x.begin(), x.end(), v) - x.begin(); }
	template <typename T, typename V>  int pos_u(const T & x, const V & v) { return upper_bound(x.begin(), x.end(), v) - x.begin(); }

	inline unsigned rand_g() {
		static mt19937 mt_rand((int)time(0));
		return mt_rand();
	}
	using LL = long long;
	using VI = vector<int>;
	using VVI = vector< VI >;
	using VL = vector<LL>;
	using PII = pair<int, int>;
	using VPII = vector< PII >;
}
#define Rep(i, a) for(int i = 0; i < (int)(a); i++)
#define Loop(i, a, b) for(int i = (int)(a); i < (int)(b); i++)
#define Rep_r(i, n) for(int i = (int)(n) - 1; i >= 0; i--)
#define Size(v) ((int)v.size())

using namespace Bigcat;
const LL base = LL(1e9 + 7);
const LL INFL = 1LL << 62;
const int INF = 1 << 30;
const int MAX = int(2e5 + 100);

struct Node {
	int x, y, w;
};
bool operator < (const Node & n0, const Node & n1) {
	return n0.x< n1.y;
}

void solve1() {

	LL result = 0;
	int n, k;

	cin >> n >>k;
	if (k == 1) {
		cout << 0 << endl;
		return;
	}
	else if (k == 2) {
		cout << (n - 2) << endl;
		return;
	}
	for (int i = 1; i <= n; ++i) {
		for (int j = 1; j <= i; ++j) { 
			int t = n - i - j;
			if (t >= k - 2) {
				max_s(result, LL(i - j));
			}
		}
	}



   cout << result << endl;

}
const bool _multi_test = false;
const int _inId = 1;
const int _outId = 0;
void io_init(char * s0) {
	string path(s0);
	int a = path.find("build\\");
	int b = path.find(
#ifdef _DEBUG
        "Debug\\"
#else
        "Release\\"
#endif
    );
	if (a > 0 && b > a) {
		path = path.substr(0, a) + path.substr(a + 6, b - a - 6);
	}
	else path = "";

	if (_inId>0) {
		cerr << "redirect stdin to input " << _inId << endl;
		string s = "input";
		s += to_string(_inId);
		s += ".txt";
		if (path != "") s = path + s;
		auto r = freopen(s.c_str(), "r", stdin);
		if (r == nullptr) {
			cerr << "invalid input file " << s << endl;;
			r = freopen("CON", "r", stdin); // to do  con??
		}
	}
	if (_outId>0) {
		cerr << "redirect stdout to stdout " << _outId << endl;
		string s = "stdout";
		s += to_string(_outId);
		s += ".txt";
		if (path != "") s = path + s;
		auto r = freopen(s.c_str(), "w", stdout);
		if (r == nullptr) {
			cerr << "invalid output file " << s << endl;;
			r = freopen("CON", "w", stdout);
		}
	} 
}

const bool _local_test =
#ifdef _LOCAL_TEST
true;
#else 
false;
#endif
void solve() {
	int t = 1;
	if (_multi_test) cin >> t;
    if (_local_test) cerr << " now base " << base << endl;
	for (int i = 0; i < t; ++i) {
        if (_local_test) cerr << " tasks: " << i + 1 << endl;
        //cout << "Case #" << i + 1 << ": ";
		solve1();
	}
	int x = 1;
}

signed main(signed argc, char * argv[]) {

    if (_local_test) {
        if (argc == 1) {
            io_init(argv[0]);
        }
    }
	cin.sync_with_stdio(0); cin.tie(0); cout.tie(0); // close sync between cout and printf
    cin.exceptions(cin.failbit);
	cout.precision(11);
	solve();
}


Measured memory_bytes: 256