hackerrank

import java.io.*;
import java.util.*;
import java.text.*;
import java.math.*;
import java.util.regex.*;

public class Solution {

    public static void main(String[] args) {
        Scanner in = new Scanner(System.in);
        int n = in.nextInt();
        int p = in.nextInt();
        // your code goes here
        int maxtime = n/2;
        int usedtime = p/2;
        if(p <=1 || p == n){
            System.out.println(0);
        }else if(usedtime > (maxtime/2)){ 
            System.out.println(maxtime - usedtime);
        } else { 
            System.out.println(usedtime);
        }
    }
}