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 java.io.*;
import java.util.*;
import java.text.*;
import java.lang.*;
import java.lang.reflect.Array;
import java.math.BigInteger;
import java.util.regex.*;
public class Myclass {
		public static ArrayList a[]=new ArrayList[300001];
		/*static boolean visited[]=new boolean [300001];
		static boolean trace=false;
		static int f,b,ans=0,fi,extra;
		static void dfs(int n) {
		*/
		
		public static void main(String[] args) 
		{
		    InputReader in = new InputReader(System.in);
			PrintWriter pw = new PrintWriter(System.out);
			long n=in.nextLong();
			long m=in.nextLong();
			long d=m;
			if(m%2==0)
				m--;
			if(n+(n-1)<m)
					pw.println(0);
			else
			{
				if(n>=m) {
					pw.println((m+1)/2-1);	
				}
				else
				{
					 long mini=(m+1)/2;
					 if(n<mini) {
						 pw.println(0);
					 }
					 else
					 {
						 if(d%2==0)
						 pw.println(n-mini);
						 else
							 pw.println(n-mini+1);
 
					 }
				}
			}
			pw.flush();
			pw.close();
		
		}
		
		static class InputReader 
		{
		private final InputStream stream;
		private final byte[] buf = new byte[8192];
		private int curChar, snumChars;
		private SpaceCharFilter filter;
 
		public InputReader(InputStream stream) 
		{
			this.stream = stream;
		}
		public int snext() 
		{
			if (snumChars == -1)
				throw new InputMismatchException();
			if (curChar >= snumChars) 
			{
				curChar = 0;
				try 
				{
					snumChars = stream.read(buf);
				} 
				
				catch (IOException e) 
				{
					throw new InputMismatchException();
				}
				
				if (snumChars <= 0)
					return -1;
			}
			return buf[curChar++];
		}
 
		public int nextInt() 
	    {
			int c = snext();
			while (isSpaceChar(c)) 
			{
				c = snext();
			}
			int sgn = 1;
			if (c == '-')
		    {
				sgn = -1;
				c = snext();
			}
			int res = 0;
			do 
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public long nextLong()
	    {
			int c = snext();
			while (isSpaceChar(c)) 
			{
				c = snext();
			}
			int sgn = 1;
			if (c == '-') 
			{
				sgn = -1;
				c = snext();
			}
			long res = 0;
			do 
			{
				if (c < '0' || c > '9')
					throw new InputMismatchException();
				res *= 10;
				res += c - '0';
				c = snext();
			} while (!isSpaceChar(c));
			return res * sgn;
		}
 
		public int[] nextIntArray(int n) 
		{
			int a[] = new int[n];
			for (int i = 0; i < n; i++) 
			{
				a[i] = nextInt();
			}
			return a;
		}
		static class tri implements Comparable<tri> {
			int p, c, l;
 
			tri(int p, int c, int l) {
				this.p = p;
				this.c = c;
				this.l = l;
			}
 
			public int compareTo(tri o) {
				return Integer.compare(l, o.l);
			}
		}
 
		public String readString()
	    {
			int c = snext();
			while (isSpaceChar(c)) 
			{
				c = snext();
			}
			StringBuilder res = new StringBuilder();
			do 
			{
				res.appendCodePoint(c);
				c = snext();
			} while (!isSpaceChar(c));
			return res.toString();
		}
 
		public String nextLine() 
		{
			int c = snext();
			while (isSpaceChar(c))
				c = snext();
			StringBuilder res = new StringBuilder();
			do 
			{
				res.appendCodePoint(c);
				c = snext();
			} while (!isEndOfLine(c));
			return res.toString();
		}
 
		public boolean isSpaceChar(int c) 
		{
			if (filter != null)
				return filter.isSpaceChar(c);
			return c == ' ' || c == '\n' || c == '\r' || c == '\t' || c == -1;
		}
 
		private boolean isEndOfLine(int c) 
		{
			return c == '\n' || c == '\r' || c == -1;
		}
 
		public interface SpaceCharFilter
	    {
			public boolean isSpaceChar(int ch);
		}
	}
		public static long mod = 1000000007;
		public static int d;
		public static int p;
		public static int q;
		
		public static int[] suffle(int[] a,Random gen)
		{
			int n = a.length;
			for(int i=0;i<n;i++)
			{
				int ind = gen.nextInt(n-i)+i;
				int temp = a[ind];
				a[ind] = a[i];
				a[i] = temp;
			}
			return a;
		}
		
		public static void swap(int a, int b){
			int temp = a;
			a = b;
			b = temp;
		}
		
		public static HashSet<Integer> primeFactorization(int n)
		{
			HashSet<Integer> a =new HashSet<Integer>();
			for(int i=2;i*i<=n;i++)
			{
				while(n%i==0)
				{
					a.add(i);
					n/=i;
				}
			}
			if(n!=1)
				a.add(n);
			return a;
		}
		
		public static void sieve(boolean[] isPrime,int n)
		{
			for(int i=1;i<n;i++)
				isPrime[i] = true;
			
			isPrime[0] = false;
			isPrime[1] = false;
			
			for(int i=2;i*i<n;i++)
			{
				if(isPrime[i] == true)
				{
					for(int j=(2*i);j<n;j+=i)
						isPrime[j] = false;
				}
			}
		}
		
		public static int GCD(int a,int b)
		{
			if(b==0)
				return a;
			else
				return GCD(b,a%b);
		}
		
		public static long GCD(long a,long b)
		{
			if(b==0)
				return a;
			else
				return GCD(b,a%b);
		}
		
		public static void extendedEuclid(int A,int B)
		{
			if(B==0)
			{
				d = A;
				p = 1 ;
				q = 0;
			}
			else
			{
				extendedEuclid(B, A%B);
				int temp = p;
				p = q;
				q = temp - (A/B)*q;
			}
		}
		
		public static long LCM(long a,long b)
		{
			return (a*b)/GCD(a,b);
		}
		
		public static int LCM(int a,int b)
		{
			return (a*b)/GCD(a,b);
		}
		
		public static int binaryExponentiation(int x,int n)
		{
		    int result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=result * x;
		        x=x*x;
		        n=n/2;
		    }
		    return result;
		}
		
		public static long binaryExponentiation(long x,long n)
		{
		    long result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=result * x;
		        x=x*x;
		        n=n/2;
		    }
		    return result;
		}
		
		public static int modularExponentiation(int x,int n,int M)
		{
		    int result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=(result * x)%M;
		        x=(x%M*x)%M;
		        n=n/2;
		    }
		    return result;
		}
		
		public static long modularExponentiation(long x,long n,long M)
		{
		    long result=1;
		    while(n>0)
		    {
		        if(n % 2 ==1)
		            result=(result%M * x%M)%M;
		        x=(x%M * x%M)%M;
		        n=n/2;
		    }
		    return result;
		}
		
		public static long modInverse(int A,int M)
		{
		    return modularExponentiation(A,M-2,M);
		}
		
		public static long modInverse(long A,long M)
		{
		    return modularExponentiation(A,M-2,M);
		}
		
		public static boolean isPrime(int n)
		{
		    
		    if (n <= 1)  return false;
		    if (n <= 3)  return true;
		    
		    if (n%2 == 0 || n%3 == 0) 
		    	return false;
		 
		    for (int i=5; i*i<=n; i=i+6)
		    {
		        if (n%i == 0 || n%(i+2) == 0)
		           return false;
		    }
		    
		    return true;
		}
		
		 public static long[] shuffle(long[] a, Random gen){ 
		        for(int i = 0, n = a.length;i < n;i++){ 
		            int ind = gen.nextInt(n-i)+i; 
		            long d = a[i]; 
		            a[i] = a[ind]; 
		            a[ind] = d; 
		        } 
		        return a; 
		    }
		static class pair implements Comparable<pair>
		{
			Long x;
			Integer y;
			pair(long a,int m)
			{
				this.x=a;
				this.y=m;
			}
 

 
 
 
 
 
		public int compareTo(pair o) {
				int result = x.compareTo(o.x);
				if(result==0)
					result = y.compareTo(o.y);
				
			return result;
			}  
			
			public String toString()
			{
				return x+" "+y;
			}
			
			public boolean equals(Object o)
			{
				if (o instanceof pair)
			    {
					pair p = (pair)o;
					return p.x == x && p.y == y ;
				}
				return false;
			}
			
			public int hashCode()
			{
				return new Long(x).hashCode()*31 + new Long(y).hashCode();
			}
		}
		static  class triplet
		{
			Long x;
			Long y;
			Integer z;
			triplet(long x,long y,int z)
			{
				this.x=x;
				this.y=y;
				this.z=z;
			}
		}
		static class jodi
		{
			Integer x;
			Integer y;
			jodi(int x,int y)
			{
				this.x=x;
				this.y=y;
			}
		}
} 