source_code: /* package whatever; // don't place package name! */
  
  import java.util.*;
  import java.lang.*;
  import java.io.*;
  
  /* Name of the class has to be "Main" only if the class is public. */
  public class Ideone
  {
  	public static void main (String[] args) throws java.lang.Exception
  	{
  		// your code goes here
  			long n,s,p;
  			Scanner in=new Scanner(System.in);
  			n=in.nextLong();
  			s=in.nextLong();
  			if(n==1 && s<=1)
  			{
  			    System.out.print(n-1);
  			}
  	        else if(s<n)
  	        {
  	            if(s%2!=0)
  		        {System.out.print(s/2);}
  		        else
  		        {System.out.print(s/2-1);}
  	        }
  	        else if(s==n)
  	{
  	    if(s%2==0)
  		{System.out.println((n/2)-1);}
  		else
  		{System.out.println(n/2);}
  	}
  	else if(s<=(2*n-1))	
  	{
  		System.out.print((2*n+1-s)/2);
  	}
  	else
  	{
  		System.out.print(0);
  	}
  	}
  }