Tanechka is shopping in the toy shop. There are exactly n toys in the shop for sale, the cost of the i-th toy is i burles. She wants to choose two toys in such a way that their total cost is k burles. How many ways to do that does she have?

Each toy appears in the shop exactly once. Pairs (a, b) and (b, a) are considered equal. Pairs (a, b), where a=b, are not allowed.

Input

The first line of the input contains two integers n, k (1 ≤ n, k ≤ 10^{14}) — the number of toys and the expected total cost of the pair of toys.

Output

Print the number of ways to choose the pair of toys satisfying the condition above. Print 0, if Tanechka can choose no pair of toys in such a way that their total cost is k burles.

Examples

Input

8 5


Output

2


Input

8 15


Output

1


Input

7 20


Output

0


Input

1000000000000 1000000000001


Output

500000000000

Note

In the first example Tanechka can choose the pair of toys (1, 4) or the pair of toys (2, 3).

In the second example Tanechka can choose only the pair of toys (7, 8).

In the third example choosing any pair of toys will lead to the total cost less than 20. So the answer is 0.

In the fourth example she can choose the following pairs: (1, 1000000000000), (2, 999999999999), (3, 999999999998), ..., (500000000000, 500000000001). The number of such pairs is exactly 500000000000.
import static java.lang.Math.*;

public class B {

	public Object solve() {
		long N = sc.nextLong(), K = sc.nextLong();

		long Q = (K-1)/2, R = max(0, N - K/2);
		long res = min(Q, R);
		exit(res);
		return null;
	}

	private static final boolean ONE_TEST_CASE = true;

	private static void init() {
	}

	////////////////////////////////////////////////////////////////////////////////////
	private static IOUtils.MyScanner sc = new IOUtils.MyScanner();
	private static Object print (Object o, Object ... A) { IOUtils.print(o, A); return null; }
	private static Object exit (Object o, Object ... A) { print(o, A); IOUtils.exit(); return null; }
	private static class IOUtils {
		public static class MyScanner {
			public String next() { newLine(); return line[index++]; }
			public int nextInt() { return Integer.parseInt(next()); }
			public long nextLong() { return Long.parseLong(next()); }
			//////////////////////////////////////////////
			private boolean eol() { return index == line.length; }
			private String readLine() {
				try {
					return r.readLine();
				} catch (Exception e) {
					throw new Error (e);
				}
			}
			private final java.io.BufferedReader r;
			private MyScanner () { this(new java.io.BufferedReader(new java.io.InputStreamReader(System.in))); }
			private MyScanner (java.io.BufferedReader r) {
				try {
					this.r = r;
					while (!r.ready())
						Thread.sleep(1);
					start();
				} catch (Exception e) {
					throw new Error(e);
				}
			}
			private String [] line;
			private int index;
			private void newLine() {
				if (line == null || eol()) {
					line = split(readLine());
					index = 0;
				}
			}
			private String [] split(String s) { return s.length() > 0 ? s.split(" ") : new String [0]; }
		}
		private static String build(Object o, Object ... A) { return buildDelim(" ", o, A); }
		private static String buildDelim(String delim, Object o, Object ... A) {
			StringBuilder b = new StringBuilder();
			append(b, o, delim);
			for (Object p : A)
				append(b, p, delim);
			return b.substring(delim.length());
		}
		//////////////////////////////////////////////////////////////////////////////////
		private static final java.text.DecimalFormat formatter = new java.text.DecimalFormat("#.#########");
		private static void start() { if (t == 0) t = millis(); }
		private static void append(java.util.function.Consumer<Object> f, java.util.function.Consumer<Object> g, final Object o) {
			if (o.getClass().isArray()) {
				int len = java.lang.reflect.Array.getLength(o);
				for (int i = 0; i < len; ++i)
					f.accept(java.lang.reflect.Array.get(o, i));
			}
			else if (o instanceof Iterable<?>)
				((Iterable<?>)o).forEach(f::accept);
			else
				g.accept(o instanceof Double ? formatter.format(o) : o);
		}
		private static void append(final StringBuilder b, Object o, final String delim) {
			append(x -> { append(b, x, delim); }, x -> b.append(delim).append(x), o);
		}
		private static java.io.PrintWriter pw = new java.io.PrintWriter(System.out);
		private static Object print(Object o, Object ... A) { pw.println(build(o, A)); return null; }
		private static void err(Object o, Object ... A) { System.err.println(build(o, A)); }
		private static boolean PRINT;
		private static void write(Object o) {
			err(o, '(', time(), ')');
			if (PRINT)
				pw.println(o);
		}
		private static void exit() {
			IOUtils.pw.close();
			System.out.flush();
			err("------------------");
			err(time());
			System.exit(0);
		}
		private static long t;
		private static long millis() { return System.currentTimeMillis(); }
		private static String time() { return "Time: " + (millis() - t) / 1000.0; }
		private static void run(int N) {
			try { PRINT = System.getProperties().containsKey("PRINT"); }
			catch (Throwable t) {}

			for (int n = 1; n <= N; ++n) {
				Object res = new B().solve();
				if (res != null)
					write("Case #" + n + ": " + build(res));
			}
			exit();
		}
	}
	public static void main(String[] args) {
		init();
		int N = ONE_TEST_CASE ? 1 : sc.nextInt();
		IOUtils.run(N);
	}
}
