import java.io.*;

import java.util.*;

import java.text.*;

import java.math.*;

import java.util.regex.*;

public class Solution {

static int solve(int n, int p){

// Complete this function

if(p==1||p==n){

return 0;

}

if(n%2!=0&&p==(n-1)){

return 0;

}

if(n%2==0&&p==(n-1)){

return 1;

}

int a= p/2;

int b= (n-p)/2;

if(a>b){

return b;

}

else return a;

}

public static void main(String[] args) {

Scanner in = new Scanner(System.in);

int n = in.nextInt();

int p = in.nextInt();

int result = solve(n, p);

System.out.println(result);

}

}